Remove supportsMediaMIMEType()/2 override, part 1 of 3

Need to remove supportsMediaMIMEType()/2, as supportsMediaMIMEType()/3
should be used. This first part changes the definition so that /2 has
a default implementation, and /3 is required. All implementations of
WebMimeRegistry in Chromium currently override both methods.

BUG=231309

Review URL: https://codereview.chromium.org/25079004

git-svn-id: svn://svn.chromium.org/blink/trunk@158755 bbb929c8-8fbe-4397-9dbb-9b2b20218538