mediaControls.css and mediaControlsAndroid.css can only style <video> and <audio> elements

The mediaControls CSS files are dynamically loaded when encountering a <video>
or <audio> element during recalcStyle. They cannot be used to style any other type
of element because we don't mark other elements as needing a recalcStyle when loading
the sheets because we never expect them to style anything except the element going
through a recalc.

This moves the global rule into html.css where it belongs and adds a warning to
the sheets that you can't style anything in them except <video> and <audio>.

BUG=284739

Review URL: https://chromiumcodereview.appspot.com/23703005

git-svn-id: svn://svn.chromium.org/blink/trunk@157189 bbb929c8-8fbe-4397-9dbb-9b2b20218538