Add mock classes for Bus, ObjectProxy, and ExportedObject.

dbus/bus.h

Index: dbus/bus.h
diff --git a/dbus/bus.h b/dbus/bus.h
index e632e9ab09521f4e7caec2323bbc0a61a948197b..a11dd9b5defe6dc46555a4daeb042b9af76b124f 100644
--- a/dbus/bus.h
+++ b/dbus/bus.h
@@ -368,9 +368,12 @@ class Bus : public base::RefCountedThreadSafe<Bus> {
   // AssertOnOriginThread().
   virtual void AssertOnDBusThread();
 
+ protected:
+  // This is protected, so we can define sub classes.
+  virtual ~Bus();
+
  private:
   friend class base::RefCountedThreadSafe<Bus>;
-  virtual ~Bus();
 
   // Helper function used for Shutdown().
   void ShutdownInternal(OnShutdownCallback callback);