Bug 1353867 - Change WindowProxyHolder's native type to BrowsingContext. r=bzbarsky

Make the WindowProxyHolder hold a strong reference to a BrowsingContext, as in the future
we might not have a nsPIDOMWindowOuter (if the document is loaded in a different process).

Differential Revision: https://phabricator.services.mozilla.com/D12651

dom/html/HTMLObjectElement.cpp

@@ -380,7 +380,7 @@ Nullable<WindowProxyHolder> HTMLObjectElement::GetContentWindow(
   if (doc) {
     nsPIDOMWindowOuter* win = doc->GetWindow();
     if (win) {
-      return WindowProxyHolder(win);
+      return WindowProxyHolder(win->GetBrowsingContext());
     }
   }