Add a UseCounter for the HTMLMediaElement.canPlayType keySystem argument

This has been replaced in the spec by MediaKeys.isTypeSupported:

https://dvcs.w3.org/hg/html-media/raw-file/tip/encrypted-media/encrypted-media.html#extensions

Add a counter now so that we have the data needed for removal when the
new spec has been implemented.

BUG=334941

Review URL: https://codereview.chromium.org/135003003

git-svn-id: svn://svn.chromium.org/blink/trunk@165312 bbb929c8-8fbe-4397-9dbb-9b2b20218538