[CrOS Video Player] Search subtitle when loading video

Search .vtt subtitle with same name as video source,
and attach to video in native controls.

Bug: 909963
Change-Id: Ib4431de16e8a7ba78c0d9ea9e880884c5641047e
Reviewed-on: https://chromium-review.googlesource.com/c/1418320
Commit-Queue: Jazz Xu <jazzhsu@chromium.org>
Reviewed-by: Trent Apted <tapted@chromium.org>
Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Reviewed-by: Tommy Steimel <steimel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#631009}
7 files changed