Add Darwin thread.h implementation.

webrtc/BUILD.gn

 # Copyright (c) 2014 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.
 
 # TODO(kjellander): Rebase this to webrtc/build/common.gypi changes after r6330.
 
@@ skipping 15 matching lines @@
   if (build_with_mozilla) {
     defines += [ "WEBRTC_MOZILLA_BUILD" ]
   }
   if (build_with_chromium) {
     defines = [
       # TODO(kjellander): Cleanup unused ones and move defines closer to
       # the source when webrtc:4256 is completed.
       "FEATURE_ENABLE_VOICEMAIL",
       "EXPAT_RELATIVE_PATH",
       "GTEST_RELATIVE_PATH",
-      "NO_MAIN_THREAD_WRAPPING",

[kthelgason, 2017/03/28 13:40:45]

This define is also in webrtc/base/BUILD.gn, closer to where it's used. See
TODO.

       "NO_SOUND_SYSTEM",
       "WEBRTC_CHROMIUM_BUILD",
     ]
     include_dirs = [
       # The overrides must be included first as that is the mechanism for
       # selecting the override headers in Chromium.
       "../webrtc_overrides",
 
       # Allow includes to be prefixed with webrtc/ in case it is not an
       # immediate subdirectory of the top-level.
@@ skipping 454 matching lines @@
       ]
 
       deps = [
         "//base:base_java_test_support",
         "//webrtc/examples:AppRTCMobile_javalib",
         "//webrtc/sdk/android:libjingle_peerconnection_java",
       ]
     }
   }
 }