Bug 1383122 - P3. Don't set src attribute to null. r=jwwang

Setting the src attribute to null was incorrect, per spec https://www.w3.org/TR/WebIDL-1/#es-DOMString, if null is passed, this is to be treated as ToString(null), which is "null" (https://tc39.github.io/ecma262/#sec-tostring).
Hence setting src to null will cause a 404 error as it attempts to load "null".

To unload the element, recommended practice is:
Per spec:
https://html.spec.whatwg.org/multipage/media.html#best-practices-for-authors-using-media-elements
"by removing the element's src attribute and any source element descendants, and invoking the element's load() method."

MozReview-Commit-ID: 5Lq13CeDCSZ

--HG--
extra : rebase_source : 806bccb32fa0542ea3be3025a31da97bb331b29c

dom/media/mediasource/test/mediasource.js

@@ -22,9 +22,8 @@ function runWithMSE(testFunction) {
     document.body.appendChild(el);
     SimpleTest.registerCleanupFunction(function () {
       el.remove();
-      // Don't trigger load algorithm to prevent 'error' events.
-      el.preload = "none";
-      el.src = null;
+      el.removeAttribute("src");
+      el.load();
     });
 
     testFunction(ms, el);

dom/media/test/eme.js

@@ -380,8 +380,8 @@ function ProcessInitData(v, test, token, initData, sessionType) {
 function CleanUpMedia(v) {
   v.setMediaKeys(null);
   v.remove();
-  v.onerror = null;
-  v.src = null;
+  v.removeAttribute("src");
+  v.load();
 }
 
 /*

dom/media/test/file_access_controls.html

@@ -122,13 +122,13 @@ function nextTest() {
 
   if (gVideo) {
     gVideo.remove();
-    gVideo.preload = "none"; // Don't trigger load algorithm.
-    gVideo.src = null;
+    gVideo.removeAttribute("src");
+    gVideo.load();
   }
 
   gVideo = null;
-  SpecialPowers.forceGC();  
-  
+  SpecialPowers.forceGC();
+
   gVideo = createVideo();
   gVideo.expectedResult = gTests[gTestNum].result;
   gVideo.testDescription = gTests[gTestNum].description;
@@ -141,7 +141,7 @@ function nextTest() {
   gVideo.src = url;
   //dump("Starting test " + gTestNum + " at " + gVideo.src + " expecting:" + gVideo.expectedResult + "\n");
   if (!gTestedRemoved) {
-    document.body.appendChild(gVideo); 
+    document.body.appendChild(gVideo);
     // Will cause load() to be invoked.
   } else {
     gVideo.load();