Bug 1283754 - Part 3: Add setupSynchronousObserver to write synchronouse tests for MutaionObserver. r=boris

MozReview-Commit-ID: 1sV5QHVeEyw

--HG--
extra : rebase_source : 766a282f486a9b68c53771a7d56de3fe659b57ca

dom/animation/test/testcommon.js

@@ -186,3 +186,23 @@ if (opener) {
     opener.done();
   }
 }
+
+/**
+ * Return a new MutaionObserver which started observing |target| element
+ * with { animations: true, subtree: |subtree| } option.
+ * NOTE: This observer should be used only with takeRecords(). If any of
+ * MutationRecords are observed in the callback of the MutationObserver,
+ * it will raise an assertion.
+ */
+function setupSynchronousObserver(t, target, subtree) {
+   var observer = new MutationObserver(records => {
+     assert_unreached("Any MutationRecords should not be observed in this " +
+                      "callback");
+   });
+  t.add_cleanup(() => {
+    observer.disconnect();
+  });
+  observer.observe(target, { animations: true, subtree: subtree });
+  return observer;
+}
+