Bug 1052089 - Rename JunkScope to PrivilegedJunkScope and remove fallibility of singleton scope access. r=billm

These two things ended up getting mushed together in my tree.

js/ipc/JavaScriptShared.cpp

@@ -395,7 +395,7 @@ JavaScriptShared::findObjectById(JSContext *cx, uint32_t objId)
     }
 
     // If there's no TabChildGlobal, we use the junk scope.
-    JSAutoCompartment ac(cx, xpc::GetJunkScope());
+    JSAutoCompartment ac(cx, xpc::PrivilegedJunkScope());
     if (!JS_WrapObject(cx, &obj))
         return nullptr;
     return obj;