Bug 1431255 - Part V, Set the reflectors of the UA Widget DOM to UA Widget Scope r=bholley

The DOM elements within the UA Widget Shadow DOM should have its reflectors in
the UA Widget Scope. This is done by calling nsINode::IsInUAWidget() which
would check its containing shadow and its UA Widget bit.

To prevent JS access of the DOM element before it is in the
UA Widget Shadom DOM tree, various DOM methods are set to inaccessible to
UA Widget script. It would need to use the two special methods in ShadowRoot
instead to insert the DOM directly into the shadow tree.

MozReview-Commit-ID: Jz9iCaVIoij

dom/html/HTMLMediaElement.cpp

@@ -4942,11 +4942,14 @@ void
 HTMLMediaElement::AttachAndSetUAShadowRoot()
 {
   if (GetShadowRoot()) {
+    MOZ_ASSERT(GetShadowRoot()->IsUAWidget());
     return;
   }
 
   // Add a closed shadow root to host video controls
-  AttachShadowWithoutNameChecks(ShadowRootMode::Closed);
+  RefPtr<ShadowRoot> shadowRoot =
+    AttachShadowWithoutNameChecks(ShadowRootMode::Closed);
+  shadowRoot->SetIsUAWidget(true);
 }
 
 nsresult