Fix a issue in MediaSession when a media playback is completed
Fix a issue in MediaSession when a media playback is completed

This fix solves a issue in MediaSession when a playback is completed.
The issue is because when a playback is completed,
BMPM::OnPlaybackComplete() and BMPM::OnPause() will be called
consequtively, causing a crash in the logic of
MediaSession::OnPlayerPaused().

BUG=539998

Review URL: https://codereview.chromium.org/1587193002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#369753}

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