Bug 1362165: P5. Don't assume that when stalling all data has been loaded. r=gerald

The stalled event can be fired as playback is starting and data has yet to be processed.
So instead we wait for playback to reach the end.

MozReview-Commit-ID: 4W3M5tee2HD

--HG--
extra : rebase_source : 349321452f8e1cc3307e8cedb6ebf44949694661

dom/media/mediasource/test/test_MediaSource_memory_reporting.html

@@ -14,7 +14,7 @@ SimpleTest.waitForExplicitFinish();
 
 runWithMSE(function (ms, v) {
   // Test that memory reporting works once we've played a video.
-  once(v, "stalled", () => {
+  once(v, "ended", () => {
     // Grab a memory report.
     var mgr = SpecialPowers.Cc["@mozilla.org/memory-reporter-manager;1"]
           .getService(SpecialPowers.Ci.nsIMemoryReporterManager);
@@ -41,7 +41,10 @@ runWithMSE(function (ms, v) {
   // Load a webm video and play it.
   ms.addEventListener("sourceopen", () => {
     var sb = ms.addSourceBuffer("video/webm");
-    fetchAndLoad(sb, 'seek', [''], '.webm').then(() => v.play());
+    fetchAndLoad(sb, 'seek', [''], '.webm').then(function() {
+      ms.endOfStream();
+      v.play()
+    });
   });
 });