Use [StrictTypeChecking] for HTMLMediaElement.setMediaKeys

The mediaKeys argument should be nullable, but passing in some other
type of object should throw an exception.

BUG=none

Review URL: https://codereview.chromium.org/212643002

git-svn-id: svn://svn.chromium.org/blink/trunk@170354 bbb929c8-8fbe-4397-9dbb-9b2b20218538