Fix race when HTMLMediaElement.play() is called just after pause().

The consequence is that the Promise returned by play() is rejected by
the task created by pause(). The fix is to associate the tasks with a
list of promises.

BUG=593273

Review-Url: https://codereview.chromium.org/1865933002
Cr-Commit-Position: refs/heads/master@{#398370}
8 files changed