Bug 1299072: P16. Add mochitest. r=gerald

MozReview-Commit-ID: FRb74jIAyLN

--HG--
extra : rebase_source : 70004e5ae9da8ac7fc8957a2eb3c6600c60a19d7

dom/media/test/test_decode_error.html

@@ -27,6 +27,8 @@ function startTest(test, token) {
     ok(el.error, "Element 'error' attr expected to have a value");
     ok(el.error instanceof MediaError, "Element 'error' attr expected to be MediaError");
     is(el.error.code, MediaError.MEDIA_ERR_DECODE, "Expected a decode error");
+    ok(typeof el.error.message === 'string' || el.error.essage instanceof String, "Element 'message' attr expected to be a string");
+    ok(el.error.message.length > 0, "Element 'message' attr has content");
     el._sawError = true;
     manager.finished(token);
   }, false);
@@ -46,7 +48,12 @@ function startTest(test, token) {
 }
 
 SimpleTest.waitForExplicitFinish();
-SpecialPowers.pushPrefEnv({"set": [["media.cache_size", 40000]]}, beginTest);
+SpecialPowers.pushPrefEnv({
+  "set": [
+    ["media.cache_size", 40000],
+    ["dom.MediaError.message.enabled", true]
+  ]
+}, beginTest);
 function beginTest() {
   manager.runTests(gDecodeErrorTests, startTest);
 }