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..198b67d52008e8514ef773afd2c8771c777dc831 100644
--- a/webrtc/modules/audio_device/BUILD.gn
+++ b/webrtc/modules/audio_device/BUILD.gn
@@ -14,6 +14,14 @@ 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.
+    # TODO(tkchin): Remove after fixing
+    # https://bugs.chromium.org/p/webrtc/issues/detail?id=5748
+    cflags = [ "-Wno-thread-safety-analysis" ]
+  }
 }
 
 source_set("audio_device") {
@@ -148,6 +156,7 @@ source_set("audio_device") {
         "ios/voice_processing_audio_unit.mm",
       ]
       cflags += [ "-fobjc-arc" ]  # CLANG_ENABLE_OBJC_ARC = YES.
+
       libs = [
         "AudioToolbox.framework",
         "AVFoundation.framework",