Add video metric from webrtc to histograms.xml.

- the average end-to-end delay per frame for a received video stream

Metric added in: https://codereview.webrtc.org/1905563002/

BUG=

Review-Url: https://codereview.chromium.org/2364573002
Cr-Commit-Position: refs/heads/master@{#421772}
1 file changed