GN: Fix some build errors for iOS.

webrtc/modules/audio_device/BUILD.gn

Index: webrtc/modules/audio_device/BUILD.gn
diff --git a/webrtc/modules/audio_device/BUILD.gn b/webrtc/modules/audio_device/BUILD.gn
index f41501e7f8446a9445155c1ddb714583fcd892fe..324ab54a4bbdb2ef6f8f4c8a7d6398cb70c1fa4f 100644
--- a/webrtc/modules/audio_device/BUILD.gn
+++ b/webrtc/modules/audio_device/BUILD.gn
@@ -14,6 +14,12 @@ config("audio_device_config") {
     "include",
     "dummy",  # Contains dummy audio device implementations.
   ]
+  if (is_ios) {
+    # GN orders flags on a target before flags from configs. In order to be able
+    # suppress the -Wthread-safety-analysis warning, it has come from a config
+    # and can't be on the target directly.
+    cflags = [ "-Wno-thread-safety-analysis" ]

[kjellander_webrtc, 2016/04/06 17:12:35]

Zeke: do you think a bug should be filed for this, in case the error is
something you want to take a look at?
It looked like this:

../../webrtc/modules/audio_device/ios/objc/RTCAudioSession.mm:156:3: error:
cannot resolve lock expression [-Werror,-Wthread-safety-analysis]
  _crit.Enter();
  ^~~~~
../../webrtc/modules/audio_device/ios/objc/RTCAudioSession.mm:163:7: error:
cannot resolve lock expression [-Werror,-Wthread-safety-analysis]
  if (_crit.TryEnter()) {
      ^~~~~
../../webrtc/modules/audio_device/ios/objc/RTCAudioSession.mm:167:5: error:
cannot resolve lock expression [-Werror,-Wthread-safety-analysis]
    _crit.Leave();
    ^~~~~
../../webrtc/modules/audio_device/ios/objc/RTCAudioSession.mm:168:5: error:
cannot resolve lock expression [-Werror,-Wthread-safety-analysis]
    _crit.Leave();
    ^~~~~
4 errors generated.

[tkchin_webrtc, 2016/04/06 19:23:17]

File a bug. Should be suppressed manually in the implementation file. These
warnings are benign.

+  }
 }
 
 source_set("audio_device") {
@@ -148,6 +154,7 @@ source_set("audio_device") {
         "ios/voice_processing_audio_unit.mm",
       ]
       cflags += [ "-fobjc-arc" ]  # CLANG_ENABLE_OBJC_ARC = YES.
+
       libs = [
         "AudioToolbox.framework",
         "AVFoundation.framework",