Bug 1264182: Hide URL.createObjectURL from ServiceWorker r=mrbkap,asuth

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. (https://github.com/slightlyoff/ServiceWorker/issues/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
https://github.com/w3c/media-source/issues/168#issuecomment-410269436).

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

--HG--
extra : moz-landing-system : lando

dom/webidl/URL.webidl

@@ -40,6 +40,7 @@ interface URL {
   USVString toJSON();
 };
 
+[Exposed=(Window,DedicatedWorker,SharedWorker)]
 partial interface URL {
   [Throws]
   static DOMString createObjectURL(Blob blob);
@@ -47,10 +48,8 @@ partial interface URL {
   static void revokeObjectURL(DOMString url);
   [ChromeOnly, Throws]
   static boolean isValidURL(DOMString url);
-};
 
-// https://dvcs.w3.org/hg/html-media/raw-file/default/media-source/media-source.html
+  // https://dvcs.w3.org/hg/html-media/raw-file/default/media-source/media-source.html
-partial interface URL {
   [Throws]
   static DOMString createObjectURL(MediaSource source);
 };

testing/web-platform/tests/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')