Merge MediaControlsChromium into MediaControls

To the largest extent possible, code was copied from
MediaControlsChromium.* to MediaControls.* without modification. Where
MediaControlsChromium virtual functions called the MediaControls base
functions, the order was preserved to avoid changing behavior.

BUG=341813

Review URL: https://codereview.chromium.org/157403002

git-svn-id: svn://svn.chromium.org/blink/trunk@166753 bbb929c8-8fbe-4397-9dbb-9b2b20218538