Trace encoding/decoding time in a generic way.

Removes VP8::Encode trace in favor of VCMGenericEncoder ones and adds
one to InitEncode. Also adds an instant event to ::Encoded since this
can be done on a different thread.

Also adds the corresponding traces to VCMGenericDecoder.

BUG=webrtc:5167
R=stefan@webrtc.org

Review URL: https://codereview.webrtc.org/1412573010

Cr-Commit-Position: refs/heads/master@{#10674}

webrtc/modules/video_coding/codecs/vp8/vp8_impl.cc

@@ -716,8 +716,6 @@ uint32_t VP8EncoderImpl::MaxIntraTarget(uint32_t optimalBuffersize) {
 int VP8EncoderImpl::Encode(const VideoFrame& frame,
                            const CodecSpecificInfo* codec_specific_info,
                            const std::vector<FrameType>* frame_types) {
-  TRACE_EVENT1("webrtc", "VP8::Encode", "timestamp", frame.timestamp());
-
   if (!inited_)
     return WEBRTC_VIDEO_CODEC_UNINITIALIZED;
   if (frame.IsZeroSize())