Bug 1353867 - Change WindowProxy type. r=bzbarsky

Add a WindowProxyHolder type and generate binding code that takes or returns it whenever
the WebIDL refers to the WindowProxy type. This patch just makes the WindowProxyHolder
hold a strong reference to a nsPIDOMWindowOuter.

Differential Revision: https://phabricator.services.mozilla.com/D12650

dom/html/HTMLObjectElement.h

@@ -16,6 +16,9 @@ namespace mozilla {
 namespace dom {
 
 class HTMLFormSubmission;
+template <typename T>
+struct Nullable;
+class WindowProxyHolder;
 
 class HTMLObjectElement final : public nsGenericHTMLFormElement,
                                 public nsObjectLoadingContent,
@@ -121,7 +124,7 @@ class HTMLObjectElement final : public nsGenericHTMLFormElement,
   }
   using nsObjectLoadingContent::GetContentDocument;
 
-  nsPIDOMWindowOuter* GetContentWindow(nsIPrincipal& aSubjectPrincipal);
+  Nullable<WindowProxyHolder> GetContentWindow(nsIPrincipal& aSubjectPrincipal);
 
   using nsIConstraintValidation::GetValidationMessage;
   using nsIConstraintValidation::SetCustomValidity;