Make the type argument to HTMLMediaElement.canPlayType non-optional

http://www.whatwg.org/specs/web-apps/current-work/multipage/the-video-element.html#htmlmediaelement

Ad-hoc testing shows that it is also non-optional in Firefox Nightly,
IE11 and Opera 12.16.

BUG=334660

Review URL: https://codereview.chromium.org/138063006

git-svn-id: svn://svn.chromium.org/blink/trunk@165153 bbb929c8-8fbe-4397-9dbb-9b2b20218538