Consistently lock RTCVideoDecoder::state_.

RTCVideoDecoder::ResetInternal() forgot to lock it, which caused TSan
errors in the unittest.

Review-Url: https://codereview.chromium.org/2339043002
Cr-Commit-Position: refs/heads/master@{#418669}

content/renderer/media/gpu/rtc_video_decoder.cc

@@ -672,8 +672,10 @@ void RTCVideoDecoder::ResetInternal() {
     vda_->Reset();
   } else {
     CreateVDA(vda_codec_profile_, nullptr);
-    if (vda_)
+    if (vda_) {
+      base::AutoLock auto_lock(lock_);
       state_ = INITIALIZED;
+    }
   }
 }