Encrypted Media: Implement MediaKeys::IsTypeSupported().

The Chromium side change is at: https://codereview.chromium.org/173293002/

BUG=224791,286036
TEST=Manually tested with the Chromium side change. Will add layout test in a
later CL.

Review URL: https://codereview.chromium.org/170783017

git-svn-id: svn://svn.chromium.org/blink/trunk@167952 bbb929c8-8fbe-4397-9dbb-9b2b20218538