defeat cors attacks on audio/video tags

Neutralize error messages and fire no progress events
until media metadata has been loaded for media loaded
from cross-origin locations.

Bug: 828265, 826187
Change-Id: Iaf15ef38676403687d6a913cbdc84f2d70a6f5c6
Reviewed-on: https://chromium-review.googlesource.com/1015794
Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Commit-Queue: Fredrik Hubinette <hubbe@chromium.org>
Cr-Commit-Position: refs/heads/master@{#557312}
bool HTMLMediaElement::HasSelectedVideoTrack() {
  DCHECK(RuntimeEnabledFeatures::BackgroundVideoTrackOptimizationEnabled());

  return video_tracks_ && video_tracks_->selectedIndex() != -1;
}

bool HTMLMediaElement::HasSelectedVideoTrack() {
  DCHECK(RuntimeEnabledFeatures::BackgroundVideoTrackOptimizationEnabled());

  return video_tracks_ && video_tracks_->selectedIndex() != -1;
}
