Report the time of a video's first displayed frame for LCP

[chromium-wpt-export-bot]

This CL plumbs the information needed to capture LCP from videos into Video timing information for LCP into the HTMLVideoElement, from where it is sent to the PaintTimingDetector when the initial frame is displayed.

Video byte size is taken from the WebMediaPlayer's PipelineStatistics, while the result of the Timing-Allow-Origin check goes through URLIndex, in the same manner as CORS checks.

Change-Id: I9e8ed63a2913d5dc70b0f1b2bf9f928db4f1b908

Reviewed-on: https://chromium-review.googlesource.com/3814106 WPT-Export-Revision: 448e729815aca6964c6c10ce8f7bc48a2c843f5f