Support pausing media when a context is frozen.

Media is resumed when the context is unpaused. This feature will be used
for bfcache and pausing iframes feature policy.

BUG=907125

Change-Id: Ic3925ea1a4544242b7bf0b9ad8c9cb9f63976bbd
Reviewed-on: https://chromium-review.googlesource.com/c/1410126
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Cr-Commit-Position: refs/heads/master@{#623319}
void HTMLMediaElement::ScheduleNotifyPlaying() {
  ScheduleEvent(event_type_names::kPlaying);
  ScheduleResolvePlayPromises();
}

void HTMLMediaElement::ScheduleNotifyPlaying() {
  ScheduleEvent(event_type_names::kPlaying);
  ScheduleResolvePlayPromises();
}
