Remove MediaControls::bufferingProgressed()

The implementation just updates the position on the timeline, which
playbackProgressed() already does. This callback would make sense if
buffering progress were actually shown, but until someone implements
that it's cleaner to not have it.

BUG=341813

Review URL: https://codereview.chromium.org/188853002

git-svn-id: svn://svn.chromium.org/blink/trunk@168708 bbb929c8-8fbe-4397-9dbb-9b2b20218538