Move bind.h to rtc_base_approved.

This is more a correction than a change since bind.h was incorrectly not
a part of rtc_base when building with Chromium while still being used
(since it's a header file only).  The only dependencies that bind.h has,
are in rtc_base_approved, so there's no need to restrict usage of it to
rtc_base users only.

NOTRY=true
TBR=perkj
NO_DEPENDENCY_CHECKS=true

Review URL: https://codereview.webrtc.org/1911403002 .

Cr-Commit-Position: refs/heads/master@{#12474}

webrtc/base/BUILD.gn

@@ -98,6 +98,7 @@ static_library("rtc_base_approved") {
   sources = [
     "array_view.h",
     "atomicops.h",
+    "bind.h",
     "bitbuffer.cc",
     "bitbuffer.h",
     "buffer.cc",
@@ -359,7 +360,6 @@ static_library("rtc_base") {
     sources += [
       "bandwidthsmoother.cc",
       "bandwidthsmoother.h",
-      "bind.h",
       "callback.h",
       "fileutils_mock.h",
       "httpserver.cc",

webrtc/base/base.gyp

@@ -81,6 +81,7 @@
       'sources': [
         'array_view.h',
         'atomicops.h',
+        'bind.h',
         'bitbuffer.cc',
         'bitbuffer.h',
         'buffer.cc',
@@ -368,7 +369,6 @@
           'sources': [
             'bandwidthsmoother.cc',
             'bandwidthsmoother.h',
-            'bind.h',
             'callback.h',
             'fileutils_mock.h',
             'httpserver.cc',