Bug 1512655 part 2 - Assert Cu.setWantXrays is never called on system-principal scopes. r=kmag

Differential Revision: https://phabricator.services.mozilla.com/D14695

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

js/xpconnect/idl/xpccomponents.idl

@@ -458,6 +458,8 @@ interface nsIXPCComponents_Utils : nsISupports
      *
      * Enables Xray vision for same-compartment access for the compartment
      * indicated by |vscope|. All outgoing wrappers are recomputed.
+     *
+     * This must not be called on chrome (system-principal) scopes.
      */
     [implicit_jscontext]
     void setWantXrays(in jsval vscope);

js/xpconnect/src/XPCComponents.cpp

@@ -1978,9 +1978,8 @@ nsXPCComponents_Utils::SetWantXrays(HandleValue vscope, JSContext* cx) {
     return NS_ERROR_INVALID_ARG;
   }
   JSObject* scopeObj = js::UncheckedUnwrap(&vscope.toObject());
-  MOZ_DIAGNOSTIC_ASSERT(
+  MOZ_RELEASE_ASSERT(!AccessCheck::isChrome(scopeObj),
-      !mozJSComponentLoader::Get()->IsLoaderGlobal(scopeObj),
+                     "Don't call setWantXrays on system-principal scopes");
-      "Don't call Cu.setWantXrays() in a JSM that shares its global");
   JS::Compartment* compartment = js::GetObjectCompartment(scopeObj);
   CompartmentPrivate::Get(scopeObj)->wantXrays = true;
   bool ok = js::RecomputeWrappers(cx, js::SingleCompartment(compartment),