Revert 175546 "Eliminate MediaPlayer & MediaPlayerClient abstrac..."

> Eliminate MediaPlayer & MediaPlayerClient abstractions(seek, duration, poster APIs)
> 
> Bug=350571
> 
> Review URL: https://codereview.chromium.org/306123003

TBR=srirama.m@samsung.com

Review URL: https://codereview.chromium.org/314413003

git-svn-id: svn://svn.chromium.org/blink/trunk@175712 bbb929c8-8fbe-4397-9dbb-9b2b20218538