Currently, mediaControls.css is the base CSS, while
mediaControlsChromium(Android).css are applied as extra style via
RenderTheme::extraMediaControlsStyleSheet().

Merge mediaControlsChromium.css into mediaControls.css and simplify.

With the new base CSS, mediaControlsChromiumAndroid.css can be
simplified a great deal, improving readability and maintainability.

BUG=255491

Review URL: https://chromiumcodereview.appspot.com/18181010

git-svn-id: svn://svn.chromium.org/blink/trunk@154102 bbb929c8-8fbe-4397-9dbb-9b2b20218538