Rename RenderMediaControlsChromium to RenderMediaControls

The original RenderMediaControls was removed in a previous commit.

BUG=302500

Review URL: https://codereview.chromium.org/25699002

git-svn-id: svn://svn.chromium.org/blink/trunk@158724 bbb929c8-8fbe-4397-9dbb-9b2b20218538