Bug 1451340 - Don't attempt to RegisterUnresolvedElement for native anonymous content;r=smaug

The webconsole UI generates a large number of scrollbars, and considering their children as
potential Custom Elements causes a slowdown when setting innerHTML="" when devtools closes.

MozReview-Commit-ID: 2QCcXCnTHA2

--HG--
extra : rebase_source : 5226e5689195a047ebcb9903b4565df13b6b1aa0

dom/base/CustomElementRegistry.cpp

@@ -335,6 +335,12 @@ CustomElementRegistry::LookupCustomElementDefinition(JSContext* aCx,
 void
 CustomElementRegistry::RegisterUnresolvedElement(Element* aElement, nsAtom* aTypeName)
 {
+  // We don't have a use-case for a Custom Element inside NAC, and continuing
+  // here causes performance issues for NAC + XBL anonymous content.
+  if (aElement->IsInNativeAnonymousSubtree()) {
+    return;
+  }
+
   mozilla::dom::NodeInfo* info = aElement->NodeInfo();
 
   // Candidate may be a custom element through extension,