Enhance RTCUIApplicationStatusObserver thread safety.

webrtc/sdk/objc/Framework/Classes/VideoToolbox/RTCVideoDecoderH264.mm

Index: webrtc/sdk/objc/Framework/Classes/VideoToolbox/RTCVideoDecoderH264.mm
diff --git a/webrtc/sdk/objc/Framework/Classes/VideoToolbox/RTCVideoDecoderH264.mm b/webrtc/sdk/objc/Framework/Classes/VideoToolbox/RTCVideoDecoderH264.mm
index f657f9c2f12b68b1d1e95adf6370f418fb2655de..fdf7e1732cd7faed282c744b533254a17ec195a8 100644
--- a/webrtc/sdk/objc/Framework/Classes/VideoToolbox/RTCVideoDecoderH264.mm
+++ b/webrtc/sdk/objc/Framework/Classes/VideoToolbox/RTCVideoDecoderH264.mm
@@ -67,6 +67,17 @@ void decompressionOutputCallback(void *decoder,
   RTCVideoDecoderCallback _callback;
 }
 
+- (instancetype)init {
+  if (self = [super init]) {
+#if defined(WEBRTC_IOS)
+    __unused RTCUIApplicationStatusObserver *observer =
+        [RTCUIApplicationStatusObserver sharedInstance];
+#endif
+  }
+
+  return self;
+}
+
 - (void)dealloc {
   [self destroyDecompressionSession];
   [self setVideoFormat:nullptr];