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There's a situation that hits a DCHECK() in
MediaCustomControlsFullscreenDetector when the video become
fullscreen but the page navigates away before the timer fires.
This CL solves the issue by stopping the timer when the
ExecutionContext is destroyed.

This CL also moves the common parts of MockWebMediaPlayer into
EmptyWebMediaPlayer. There are still some other
MockWebMediaPlayers which will be adopted in a follow-up.

BUG=698034

Review-Url: https://codereview.chromium.org/2729613007
Cr-Commit-Position: refs/heads/master@{#455740}