Filpping screen on iOS/WebView

Hi,
I am using customized WebView in my iOS project. Flipping screen, changes controls layout and WebView(also UIWebView) size.
However, WebView content won't change, never follows the screen orientation. window.innerWidth and window.innerHeight won't change
after flipping.
If I focus some textbox and open keyboard, flipping screen changes window.innerWidth/innerHeight as expected.

Please advice me how can I change the size of WebView content to follow screen orientation as native Safari, despite of opening keyboard.
I have tried to set ScalesPageToFit, AutoresizingMask, AutosizesSubViews in WebViewRenderer#OnElementChanged, in vain.

Thanks,
Tanaka