Bug 1489858 - Disable mutation events on shadow trees. r=smaug

Differential Revision: https://phabricator.services.mozilla.com/D10747

--HG--
extra : moz-landing-system : lando

dom/base/nsContentUtils.cpp

@@ -4671,8 +4671,11 @@ nsContentUtils::HasMutationListeners(nsINode* aNode,
     return false;
   }
 
-  if (aNode->IsContent() && aNode->AsContent()->ChromeOnlyAccess()) {
-    return false;
+  if (aNode->IsContent()) {
+    if (aNode->AsContent()->ChromeOnlyAccess() ||
+        aNode->AsContent()->IsInShadowTree()) {
+      return false;
+    }
   }
 
   doc->MayDispatchMutationEvent(aTargetForSubtreeModified);

testing/web-platform/meta/shadow-dom/untriaged/events/test-001.html.ini

@@ -1,4 +0,0 @@
-[test-001.html]
-  [A_05_00_01_T1]
-    expected: FAIL
-