Revert 175508 "Eliminate MediaPlayer & MediaPlayerClient abstrac..."

> Eliminate MediaPlayer & MediaPlayerClient abstractions(play/pause, other APIs)
> 
> BUG=350571
> 
> Review URL: https://codereview.chromium.org/298093004

TBR=srirama.m@samsung.com

Review URL: https://codereview.chromium.org/325513002

git-svn-id: svn://svn.chromium.org/blink/trunk@175713 bbb929c8-8fbe-4397-9dbb-9b2b20218538