Eliminate MediaPlayer & MediaPlayerClient abstractions(rate, setRate and other APIs)

BUG=350571

Chromium side patch for this is https://codereview.chromium.org/303403002/

Review URL: https://codereview.chromium.org/304353005

git-svn-id: svn://svn.chromium.org/blink/trunk@175683 bbb929c8-8fbe-4397-9dbb-9b2b20218538