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}

webrtc/modules/audio_device/ios/audio_device_ios.h

@@ -92,7 +92,8 @@ class AudioDeviceIOS : public AudioDeviceGeneric,
   // 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;

webrtc/modules/audio_device/ios/voice_processing_audio_unit.mm

@@ -370,10 +370,12 @@ void VoiceProcessingAudioUnit::DisposeAudioUnit() {
       case kStarted:
         Stop();
         // Fall through.
+        FALLTHROUGH();
       case kInitialized:
         Uninitialize();
         break;
       case kUninitialized:
+        FALLTHROUGH();
       case kInitRequired:
         break;
     }