Hide URL.createObjectURL from ServiceWorker

The appropriate lifetime for URLs created with URL.createObjectURL turned
out to be tricky to define, so it was decided to hide it from service
workers altogether. (w3c/ServiceWorker#688)

This commit implements this change and adds a web platform test to verify it.
It also exposes the MediaSource variant of URL.createObjectURL in DedicatedWorker
and SharedWorker contexts in order to comply with the WebIDL spec (see
w3c/media-source#168 (comment)).

Differential Revision: https://phabricator.services.mozilla.com/D2728

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1264182
gecko-commit: 40d4999db3191bca0aeb75a50b37537ad78102ed
gecko-integration-branch: autoland
gecko-reviewers: mrbkap, asuth

service-workers/service-worker/resources/interfaces-worker.sub.js

@@ -93,3 +93,7 @@ test(function() {
 test(() => {
     assert_false('XMLHttpRequest' in self);
   }, 'xhr is not exposed');
+
+test(() => {
+    assert_false('createObjectURL' in self.URL);
+  }, 'URL.createObjectURL is not exposed')