Remove supportsMediaMIMEType()/2 override, part 3 of 3

Final step to remove supportsMediaMIMEType()/2 now that all overrides
have been removed from the Chrome code.

BUG=231309

Review URL: https://codereview.chromium.org/26251006

git-svn-id: svn://svn.chromium.org/blink/trunk@159348 bbb929c8-8fbe-4397-9dbb-9b2b20218538