More cleanups in FrameStats

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}
24 files changed