Remove OVERRIDE for overridden blink::WebMediaPlayer methods.

These OVERRIDEs will make changing blink API hard. See discussions here:
https://groups.google.com/a/chromium.org/d/topic/chromium-dev/FF0hBbv3_fM/discussion

R=acolwell@chromium.org

Review URL: https://codereview.chromium.org/106563004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242914 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242914