Summary:
@public
The `WKWebView` class doesn't expose a `scalesPageToFit` property, unlike `UIWebView`. Therefore, the `scalesPageToFit` RN prop is be a bit tricky to implement with `WKWebView`.
For the time being, this diff adds warnings to `<WebView/>` whenever `useWebKit={true}` and `scalesPageToFit` is set. I've also updated the documentation to reflect that we don't support `scalesPageToFit` prop with the new implementation of `<WebView/>`.
Reviewed By: shergin
Differential Revision: D6429271
fbshipit-source-id: adf858cb67ba221c70d6d6f1bd6cff505e90c365
b18fddadfe