Bug 1355746 - Part 4. Fix intermittent full screen test failures. r=smaug,xidorn

We found that a window will not get focus immediately after exiting full screen
mode on Linux. This seems to be a long-standing issue which surfaces due to the
change of background HTML parsing timing. So, we try to get focus everytime
before requesting full screen mode to ensure the request will not fail because
of the focus issue.

MozReview-Commit-ID: 2pOShFZcq8A

--HG--
extra : rebase_source : 47a9431e02549b483874ddfba804bed0d1c6a534
extra : intermediate-source : 64f74f99771510bc06aaf4e9fc875e61b4c67a75
extra : source : 57f259680880504181191c5fe5fa9688c0692703

dom/html/test/file_fullscreen-api.html

@@ -305,7 +305,7 @@ function testNamespaces(followupTestFn) {
       });
     }
 
-    elem.requestFullscreen();
+    SimpleTest.waitForFocus(() => elem.requestFullscreen());
   }
 
   runNextNamespaceTest();

dom/html/test/file_fullscreen-prefixed.html

@@ -103,7 +103,7 @@ class TestCase {
         }
       };
       this.changeListeners("add", eventType, handleEvent);
-      actionCallback();
+      SimpleTest.waitForFocus(() => actionCallback());
     });
   }