Reduce virtual usage in MediaControlElementTypes

mediaController() is not overridden so remove the virtual.
isMediaControlElement() can just return true instead of calling a virtual not overridden method.

Review URL: https://codereview.chromium.org/134353002

git-svn-id: svn://svn.chromium.org/blink/trunk@164902 bbb929c8-8fbe-4397-9dbb-9b2b20218538