Make MediaKeySession.release() asynchronous

release() now works in the same manner as update().

BUG=334884
TEST=Updated EME content tests pass

Review URL: https://codereview.chromium.org/173373002

git-svn-id: svn://svn.chromium.org/blink/trunk@167621 bbb929c8-8fbe-4397-9dbb-9b2b20218538