Enhance RTCUIApplicationStatusObserver thread safety.

webrtc/sdk/objc/Framework/Classes/VideoToolbox/RTCVideoEncoderH264.mm

 /*
  *  Copyright (c) 2015 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.
  *
  */
@@ skipping 283 matching lines @@
 // conditions, 0.95 seems to give us better overall bitrate over long periods
 // of time.
 - (instancetype)initWithCodecInfo:(RTCVideoCodecInfo *)codecInfo {
   if (self = [super init]) {
     _codecInfo = codecInfo;
     _bitrateAdjuster = new webrtc::BitrateAdjuster(webrtc::Clock::GetRealTimeClock(), .5, .95);
     _packetizationMode = RTCH264PacketizationModeNonInterleaved;
     _profile = ExtractProfile([codecInfo nativeVideoCodec]);
     LOG(LS_INFO) << "Using profile " << CFStringToString(_profile);
     RTC_CHECK([codecInfo.name isEqualToString:@"H264"]);
+
+#if defined(WEBRTC_IOS)
+    [RTCUIApplicationStatusObserver prepareForUse];
+#endif
   }
   return self;
 }
 
 - (void)dealloc {
   [self destroyCompressionSession];
 }
 
 - (NSInteger)startEncodeWithSettings:(RTCVideoEncoderSettings *)settings
                        numberOfCores:(int)numberOfCores {
@@ skipping 386 matching lines @@
   }
   _bitrateAdjuster->Update(frame.buffer.length);
 }
 
 - (RTCVideoEncoderQpThresholds *)scalingSettings {
   return [[RTCVideoEncoderQpThresholds alloc] initWithThresholdsLow:kLowH264QpThreshold
                                                                high:kHighH264QpThreshold];
 }
 
 @end