Add video metrics from webrtc to histograms.xml.
Add video metrics from webrtc to histograms.xml.

- the average QP per frame for VP8 for received video streams

BUG=512752

Review URL: https://codereview.chromium.org/1414243002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#355258}

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