Verify MediaKeySession release()

Now that the Chromium code has been updated to fire the close event,
update the EME events test to verify. Also removes the old EME event
interface (keyMessage, keyAdded, and keyError) which is no longer used.

BUG=224786
TEST=EME content tests pass

Review URL: https://codereview.chromium.org/140093002

git-svn-id: svn://svn.chromium.org/blink/trunk@165806 bbb929c8-8fbe-4397-9dbb-9b2b20218538