Animated scrolling on Google Maps scrolls the page in addition to moving the map

Animated scrolling on Google Maps scrolls the page in addition to moving the map
https://bugs.webkit.org/show_bug.cgi?id=192521
<rdar://problem/46382007>

Reviewed by Sam Weinig.

* Platform/spi/ios/UIKitSPI.h:
* UIProcess/ios/WKKeyboardScrollingAnimator.mm:
(-[WKKeyboardScrollViewAnimator rubberbandableDirections]):
Only do keyboard-based rubber-banding in directions that we can actually
scroll, not directions we can only finger-rubber-band in. This effectively
means keyboard scrolling will ignore "alwaysBounce{Vertical, Horizontal}".

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@239059 268f45cc-cd09-0410-ab3c-d52691b4dbfc