Fix HTMLMediaElement to always use the 'effective playback rate'

Explicitly introduce the "effective playback rate" concept
in the HTML5 spec and use it to make sure the MediaController's
playback rate is always used when a controller is attached.


BUG=385408
TEST=LayoutTests/media/media-controller-effective-playback-rate.html

Review URL: https://codereview.chromium.org/336303002

git-svn-id: svn://svn.chromium.org/blink/trunk@176344 bbb929c8-8fbe-4397-9dbb-9b2b20218538