Eliminate MediaPlayer & MediaPlayerClient abstractions

This is a chromium side patch to help eliminate MediaPlayer and
MediaPlayerClient interfaces. This helps HTMLMediaElement to
directly interact with WebMediaPlayer.

BUG=350571, 310466

Blink CL: https://codereview.chromium.org/1055503002/

Review URL: https://codereview.chromium.org/1133033003

Cr-Commit-Position: refs/heads/master@{#338641}
15 files changed