Verify EME needkey event property contentType

Now that the Chromium code has been updated to pass the MIME type
of the media file, update the needkey test to verify the value.
Also removes the old keyNeeded() interface which is no longer used.

BUG=224786
TEST=EME content tests pass

Review URL: https://codereview.chromium.org/129793008

git-svn-id: svn://svn.chromium.org/blink/trunk@165805 bbb929c8-8fbe-4397-9dbb-9b2b20218538