Merge MediaControlsAndroid into MediaControls

The additional functionality in MediaControlsAndroid is so small that
merging it was trivial. Just use CSS to hide the overlay enclosure.

One unsightly ifdef remains in MediaControls::shouldHideControls(),
hopefully this can eventually be removed by using a setting or perhaps
checking the device capabilities.

MediaControls::createTextTrackDisplay() was made simpler.

BUG=341813

Review URL: https://codereview.chromium.org/192453005

git-svn-id: svn://svn.chromium.org/blink/trunk@168967 bbb929c8-8fbe-4397-9dbb-9b2b20218538