Fix iOS GN build and cleanup system_wrappers

Compile fixes for GN on iOS that finally gets our bots green.

Changes to system_wrappers:
* Updated to only use inclusive sources for maintainability
* Add a few missing GN headers.
* Cleanup GYP hack for atomic32_mac.cc
* Renamed changes sources to avoid problems with GYP/GN file
   suffix rules:
  - atomic32_mac.cc -> atomic32_darwin.cc
  - atomic32_posix.cc -> atomic32_non_darwin_unix.cc
See https://code.google.com/p/chromium/codesearch#chromium/src/build/config/BUILDCONFIG.gn&l=325
for details on which extensions can/cannot be used.

BUG=webrtc:5586
NOTRY=True

Review-Url: https://codereview.webrtc.org/1999723002
Cr-Commit-Position: refs/heads/master@{#12897}
diff --git a/webrtc/modules/audio_device/ios/audio_device_ios.h b/webrtc/modules/audio_device/ios/audio_device_ios.h
index 9616a28..e6b72d6 100644
--- a/webrtc/modules/audio_device/ios/audio_device_ios.h
+++ b/webrtc/modules/audio_device/ios/audio_device_ios.h
@@ -92,7 +92,8 @@
   // See audio_device_not_implemented.cc for trivial implementations.
   int32_t PlayoutBuffer(AudioDeviceModule::BufferType& type,
                         uint16_t& sizeMS) const override;
-  int32_t ActiveAudioLayer(AudioDeviceModule::AudioLayer& audioLayer) const;
+  int32_t ActiveAudioLayer(
+      AudioDeviceModule::AudioLayer& audioLayer) const override;
   int32_t ResetAudioDevice() override;
   int32_t PlayoutIsAvailable(bool& available) override;
   int32_t RecordingIsAvailable(bool& available) override;
diff --git a/webrtc/modules/audio_device/ios/voice_processing_audio_unit.mm b/webrtc/modules/audio_device/ios/voice_processing_audio_unit.mm
index db7f42e..a9d599a 100644
--- a/webrtc/modules/audio_device/ios/voice_processing_audio_unit.mm
+++ b/webrtc/modules/audio_device/ios/voice_processing_audio_unit.mm
@@ -370,10 +370,12 @@
       case kStarted:
         Stop();
         // Fall through.
+        FALLTHROUGH();
       case kInitialized:
         Uninitialize();
         break;
       case kUninitialized:
+        FALLTHROUGH();
       case kInitRequired:
         break;
     }
diff --git a/webrtc/modules/utility/BUILD.gn b/webrtc/modules/utility/BUILD.gn
index 6704cd6..6cd3dbb 100644
--- a/webrtc/modules/utility/BUILD.gn
+++ b/webrtc/modules/utility/BUILD.gn
@@ -24,6 +24,7 @@
     "source/file_recorder_impl.cc",
     "source/file_recorder_impl.h",
     "source/helpers_android.cc",
+    "source/helpers_ios.mm",
     "source/jvm_android.cc",
     "source/process_thread_impl.cc",
     "source/process_thread_impl.h",
diff --git a/webrtc/modules/video_coding/BUILD.gn b/webrtc/modules/video_coding/BUILD.gn
index e0f26f3..c18ae71 100644
--- a/webrtc/modules/video_coding/BUILD.gn
+++ b/webrtc/modules/video_coding/BUILD.gn
@@ -151,6 +151,11 @@
     "../../system_wrappers",
   ]
 
+  if (is_ios) {
+    sources += [ "codecs/h264/h264_objc.mm" ]
+    deps += [ ":webrtc_h264_video_toolbox" ]
+  }
+
   if (rtc_use_h264) {
     defines += [ "WEBRTC_USE_H264" ]
     if (rtc_initialize_ffmpeg) {
@@ -170,8 +175,52 @@
   }
 }
 
-# TODO(tkchin): Source set for webrtc_h264_video_toolbox. Currently not
-# possible to add, see https://crbug.com/297668.
+if (is_ios) {
+  source_set("webrtc_h264_video_toolbox") {
+    sources = [
+      "codecs/h264/h264_video_toolbox_decoder.cc",
+      "codecs/h264/h264_video_toolbox_decoder.h",
+      "codecs/h264/h264_video_toolbox_encoder.cc",
+      "codecs/h264/h264_video_toolbox_encoder.h",
+      "codecs/h264/h264_video_toolbox_nalu.cc",
+      "codecs/h264/h264_video_toolbox_nalu.h",
+    ]
+
+    configs += [ "//build/config/compiler:enable_arc" ]
+
+    cflags = [ "-Wobjc-missing-property-synthesis" ]
+
+    precompiled_header = "../../sdk/objc/WebRTC-Prefix.pch"
+    precompiled_source = "../../sdk/objc/WebRTC-Prefix.pch"
+
+    deps = [
+      "../../sdk:rtc_sdk_common_objc",
+    ]
+
+    libs = [
+      "CoreFoundation.framework",
+      "CoreMedia.framework",
+      "CoreVideo.framework",
+      "VideoToolbox.framework",
+    ]
+
+    if (is_clang) {
+      # Suppress warnings from Chrome's Clang plugins.
+      # See http://code.google.com/p/webrtc/issues/detail?id=163 for details.
+      configs -= [ "//build/config/clang:find_bad_constructs" ]
+    }
+
+    if (rtc_build_libyuv) {
+      deps += [ "$rtc_libyuv_dir" ]
+      public_deps = [
+        "$rtc_libyuv_dir",
+      ]
+    } else {
+      # Need to add a directory normally exported by libyuv.
+      include_dirs += [ "$rtc_libyuv_dir/include" ]
+    }
+  }
+}
 
 source_set("webrtc_i420") {
   sources = [