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Until we can rework the guts of HTMLMediaElement and its interaction
with WebMediaPlayer, have WebMediaPlayerImpl track an ended state so it
can return the duration HTMLMediaElement expects to fire the ended
event.

This makes it easier to change how Pipeline's audio/video time is
implemented without worrying about the pecularities of HTMLMediaElement.

BUG=349543,370634

Review URL: https://codereview.chromium.org/526443003

Cr-Commit-Position: refs/heads/master@{#293263}