Enhance RTCUIApplicationStatusObserver thread safety.

webrtc/sdk/objc/Framework/Classes/VideoToolbox/RTCVideoDecoderH264.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 49 matching lines @@
   decodeParams->callback(decodedFrame);
 }
 
 // Decoder.
 @implementation RTCVideoDecoderH264 {
   CMVideoFormatDescriptionRef _videoFormat;
   VTDecompressionSessionRef _decompressionSession;
   RTCVideoDecoderCallback _callback;
 }
 
+- (instancetype)init {
+  if (self = [super init]) {
+#if defined(WEBRTC_IOS)
+    [RTCUIApplicationStatusObserver prepareForUse];
+#endif
+  }
+
+  return self;
+}
+
 - (void)dealloc {
   [self destroyDecompressionSession];
   [self setVideoFormat:nullptr];
 }
 
 - (NSInteger)startDecodeWithSettings:(RTCVideoEncoderSettings *)settings
                        numberOfCores:(int)numberOfCores {
   return WEBRTC_VIDEO_CODEC_OK;
 }
 
@@ skipping 163 matching lines @@
   if (_videoFormat) {
     CFRetain(_videoFormat);
   }
 }
 
 - (NSString *)implementationName {
   return @"VideoToolbox";
 }
 
 @end