Removing NIXSetAutoScaleToFitContents

It was not making any difference and was only adding more complexity to the code

Reviewed by: Lauro Neto
Adding didChangeScaleFactor callback to WebViewClient

It takes a while to the WebPageProxy (UIProcess) has its scale factor synchronized with the WebProcess,
due to IPC.
This callback is useful to let the user know when it can rely on WebView's scale factor.

Reviewed by: Lauro Neto
Removing unused callback from WebView.h

Since r161087 didChangeContentScaleFactor is not needed anymore

Reviewed by: Lauro Neto
Fixing flickering when doing pinch in a page

We must cache the scale factor before sending it to WebPageProxy directly, then when the user
ends the gesture, we can set it.

Reviewed by: Lauro Neto
Restoring zoom and scroll position when going back/forward

Also did a huge refactoring to remove unecessary and messy code

Reviewed by: Lauro Neto