[iOS] Link with base/maccocoathreadhelper.mm on iOS.

rtc::ThreadManager::ThreadManager() calls rtc::InitCocoaMultiThreading()
on iOS so add base/maccocoathreadhelper.mm to the file to build on iOS.

Fixes the following linker error:

Undefined symbols for architecture x86_64:
  "rtc::InitCocoaMultiThreading()", referenced from:
      rtc::ThreadManager::ThreadManager() in librtc_base.a(thread.o)

BUG=459705
NOTRY=True

Review URL: https://codereview.webrtc.org/1810373003

Cr-Commit-Position: refs/heads/master@{#12085}

webrtc/base/BUILD.gn

@@ -521,6 +521,8 @@ static_library("rtc_base") {
     all_dependent_configs = [ ":ios_config" ]
 
     sources += [
+      "maccocoathreadhelper.h",
+      "maccocoathreadhelper.mm",
       "macconversion.cc",
       "macconversion.h",
     ]