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}