Reland of "Decoupling rtc_base from Obj-C code"

Reason for revert:
Trying to fix the link error on the chromium FYI trybots.

Original issue's description:
> Revert of Decoupling rtc_base from Obj-C code (patchset #1 id:1 of https://codereview.webrtc.org/3004373002/ )
>
> Reason for revert:
> It breaks a chromium FYI trybot (the switch from rtc_static_library to rtc_source_set seems to be the problem).
>
> Original issue's description:
> > Reland of Decoupling rtc_base from Obj-C code (patchset #1 id:1 of https://codereview.webrtc.org/3008103002/ )
> >
> > Reason for revert:
> > Trying to reland after fixing downstream project.
> >
> > Original issue's description:
> > > Revert of Decoupling rtc_base from Obj-C code (patchset #8 id:140001 of https://codereview.webrtc.org/3001623003/ )
> > >
> > > Reason for revert:
> > > It breaks a downstream project.
> > >
> > > Original issue's description:
> > > > Decoupling rtc_base from Obj-C code
> > > >
> > > > The goal of this CL is to separate Obj-C/Obj-C++ code from targets which have
> > > > also C++ code (see https://bugs.chromium.org/p/webrtc/issues/detail?id=7743
> > > > for more information).
> > > >
> > > > To achieve this we have created 2 targets (rtc_base_objc and rtc_base_generic)
> > > > and rtc_base will act as a proxy between these targets (this way we can avoid a
> > > > circular dependency between rtc_base_generic and rtc_base_objc).
> > > >
> > > > BUG=webrtc:7743
> > > >
> > > > Review-Url: https://codereview.webrtc.org/3001623003
> > > > Cr-Commit-Position: refs/heads/master@{#19661}
> > > > Committed: a0a5b98937
> > >
> > > TBR=kjellander@webrtc.org,kwiberg@webrtc.org
> > > # Skipping CQ checks because original CL landed less than 1 days ago.
> > > NOPRESUBMIT=true
> > > NOTREECHECKS=true
> > > NOTRY=true
> > > BUG=webrtc:7743
> > >
> > > Review-Url: https://codereview.webrtc.org/3008103002
> > > Cr-Commit-Position: refs/heads/master@{#19664}
> > > Committed: 791a8f611d
> >
> > TBR=kjellander@webrtc.org,kwiberg@webrtc.org
> > # Skipping CQ checks because original CL landed less than 1 days ago.
> > NOPRESUBMIT=true
> > NOTREECHECKS=true
> > NOTRY=true
> > BUG=webrtc:7743
> >
> > Review-Url: https://codereview.webrtc.org/3004373002
> > Cr-Commit-Position: refs/heads/master@{#19681}
> > Committed: a0c893f324
>
> TBR=kjellander@webrtc.org,kwiberg@webrtc.org
> # Skipping CQ checks because original CL landed less than 1 days ago.
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=webrtc:7743
>
> Review-Url: https://codereview.webrtc.org/3007183002
> Cr-Commit-Position: refs/heads/master@{#19688}
> Committed: 9c5e511806

TBR=kjellander@webrtc.org,kwiberg@webrtc.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=webrtc:7743

BUG=webrtc:7743

Review-Url: https://codereview.webrtc.org/3012823002
Cr-Commit-Position: refs/heads/master@{#19726}

webrtc/rtc_base/BUILD.gn

@@ -428,6 +428,35 @@ rtc_source_set("rtc_json") {
 }
 
 rtc_static_library("rtc_base") {
+  public_deps = [
+    ":rtc_base_generic",
+  ]
+  if (is_win) {
+    sources = [
+      "noop.cc",
+    ]
+  }
+  if (is_ios || is_mac) {
+    sources = [
+      "noop.mm",
+    ]
+    public_deps += [ ":rtc_base_objc" ]
+  }
+}
+
+if (is_ios || is_mac) {
+  rtc_source_set("rtc_base_objc") {
+    sources = [
+      "thread_darwin.mm",
+    ]
+    deps = [
+      ":rtc_base_generic",
+    ]
+    visibility = [ ":rtc_base" ]
+  }
+}
+
+rtc_static_library("rtc_base_generic") {
   cflags = []
   cflags_cc = []
   libs = []
@@ -537,6 +566,11 @@ rtc_static_library("rtc_base") {
     "thread.h",
   ]
 
+  visibility = [
+    ":rtc_base",
+    ":rtc_base_objc",
+  ]
+
   # TODO(henrike): issue 3307, make rtc_base build with the Chromium default
   # compiler settings.
   suppressed_configs += [ "//build/config/compiler:chromium_code" ]
@@ -596,10 +630,7 @@ rtc_static_library("rtc_base") {
   }
 
   if (is_ios || is_mac) {
-    sources += [
-      "macifaddrs_converter.cc",
-      "thread_darwin.mm",
-    ]
+    sources += [ "macifaddrs_converter.cc" ]
   }
 
   if (use_x11) {

webrtc/rtc_base/noop.cc

@@ -0,0 +1,13 @@
+/*
+ *  Copyright 2015 The WebRTC Project Authors. All rights reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+// This file is only needed to make ninja happy on some platforms.
+// On some platforms it is not possible to link an rtc_static_library
+// without any source file listed in the GN target.

webrtc/rtc_base/noop.mm

@@ -0,0 +1,13 @@
+/*
+ *  Copyright 2015 The WebRTC Project Authors. All rights reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+// This file is only needed to make ninja happy on some platforms.
+// On some platforms it is not possible to link an rtc_static_library
+// without any source file listed in the GN target.