Fix -webkit-app-region: drag don't work any more once it maximized.

Part of #1168.

This feature is broken by commit da2b25c991d8c9b505fce21ac5a331057c726c25.

TEST=1. Run the nw-sample-apps\frameless-window;
            2. Show the titlebar by cliking "Top Titlebar"
            3. Double click the maximize the windows.
            4. Double click the tilte bar should be able to unmaximize the window.