Verify MediaKeys parameter values consistently

Change to use same logic for verifying keySystem and contentType values when creating MediaKeys and MediaKeySessions as is used for isTypeSupported(). Removes unfinished stub code in ContentDecryptionModule.

BUG=342107
TEST=manual

Review URL: https://codereview.chromium.org/183943003

git-svn-id: svn://svn.chromium.org/blink/trunk@168321 bbb929c8-8fbe-4397-9dbb-9b2b20218538