Bug 1366151 - rewrite dom/html/test/test_fullscreen-api-race.html. r=smaug

Run this test with security.data_uri.unique_opaque_origin set to false
so we can reply on SimpleTests.waitForFocus() to be fired.

dom/html/test/test_fullscreen-api-race.html

@@ -35,7 +35,9 @@ addLoadEvent(function () {
   SpecialPowers.pushPrefEnv({
     "set": [
       ["full-screen-api.unprefix.enabled", true],
-      ["full-screen-api.allow-trusted-requests-only", false]
+      ["full-screen-api.allow-trusted-requests-only", false],
+      // Use legacy data: URI behavior to run test.
+      ["security.data_uri.unique_opaque_origin", false]
     ]
   }, next);
 });