Since our extension system enforces that XWalkExtension superclass is always initialized before the subclass, moreover, during the initialization of XWalkExtension class, it will ask XWalkExtensionManager to register itself into extension system. Once an XWalkExtension is registered, the native counterpart is also completed to initialize. 

As a result, there is no chance for its subclass to change the JS api content.

I think this is the reason why @yinli has to do it before new ScreenOrientationExtension instead of doing it into the extension impl.