1. Separated HostFrameStats and ClientFrameStats. This will allow to
   track host and client stats separately.
2. Added FrameStatsConsumer interface to allow verify that FrameStats
   are generated correctly.
3. Added VideoRenderer::GetFrameStatsConsumer() and changed
   WebrtcVideoRendererAdapter to get VideoRenderer instead of
   FrameConsumer, so now the adapter can report FrameStats.

BUG=621691

Review-Url: https://codereview.chromium.org/2113523007
Cr-Commit-Position: refs/heads/master@{#403953}