Add UseCounters for webkitAudioDecodedByteCount/webkitVideoDecodedByteCount

BUG=309364

Review URL: https://codereview.chromium.org/28903009

git-svn-id: svn://svn.chromium.org/blink/trunk@160023 bbb929c8-8fbe-4397-9dbb-9b2b20218538