Remove codec thread from MediaCodecVideoEncoder.

After this change, all calls to MediaCodecVideoEncoder must be made on
the same task queue. Removes OnCodecThread suffix from methods since it
is no longer meaningful.

BUG=webrtc:6290

Review-Url: https://codereview.webrtc.org/2669093004
Cr-Commit-Position: refs/heads/master@{#16792}

webrtc/modules/video_coding/codecs/vp8/simulcast_encoder_adapter.cc

@@ -149,6 +149,7 @@ int SimulcastEncoderAdapter::Release() {
   while (!streaminfos_.empty()) {
     VideoEncoder* encoder = streaminfos_.back().encoder;
     EncodedImageCallback* callback = streaminfos_.back().callback;
+    encoder->Release();
     factory_->Destroy(encoder);
     delete callback;
     streaminfos_.pop_back();