Implement MediaKeySession.generateRequest()

The latest EME spec has MediaKeys.createSession() return the session
object, and adds MediaKeySession.generateRequest() to actually call
the CDM with the necessary |initData| for the session.

This change also updates the layout tests to match the new way of
creating sessions.

BUG=358271
TEST=updated layout tests pass

Review URL: https://codereview.chromium.org/543173002

git-svn-id: svn://svn.chromium.org/blink/trunk@181765 bbb929c8-8fbe-4397-9dbb-9b2b20218538