Bug 1139560 - Fix mochitest expectations for currentSrc. r=orange for a CLOSED TREE

dom/tests/mochitest/general/test_img_mutations.html

@@ -69,7 +69,7 @@
       ok(img.currentSrc.endsWith("non_existent_image.404"), "Should have synchronously selected source");
 
       img.removeAttribute("src");
-      is(img.currentSrc, null, "Should have dropped currentSrc");
+      is(img.currentSrc, '', "Should have dropped currentSrc");
 
       // Load another image while previous load is still pending
       img.src = testPNG200;

dom/tests/mochitest/general/test_picture_mutations.html

@@ -65,7 +65,7 @@
       info("test 1");
       img.srcset = testPNG100;
       img.src = testPNG50;
-      is(img.currentSrc, null, "Should not have synchronously selected source");
+      is(img.currentSrc, '', "Should not have synchronously selected source");
 
       // No events should have fired synchronously, now we should get just one load (and no 404 error)
       expectEvents(1, 0, nextTest);