Eliminate MediaPlayer abstraction(play/pause, other APIs)

BUG=350571

Review URL: https://codereview.chromium.org/370203004

git-svn-id: svn://svn.chromium.org/blink/trunk@177703 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Make some layout tests wait on events rather than setTimeout

BUG=none

Review URL: https://codereview.chromium.org/378983002

git-svn-id: svn://svn.chromium.org/blink/trunk@177704 bbb929c8-8fbe-4397-9dbb-9b2b20218538