[iOS][WK2] Implement WKPDFView smart magnification

[iOS][WK2] Implement WKPDFView smart magnification
https://bugs.webkit.org/show_bug.cgi?id=134269
<rdar://problem/17272825>

Reviewed by Dan Bernstein.

* UIProcess/API/Cocoa/WKWebView.mm:
(-[WKWebView _currentContentView]):
(contentZoomScale):
Factor _currentContentView out of contentZoomScale.

(-[WKWebView _zoomToRect:WebCore::atScale:origin:WebCore::]):
(-[WKWebView _scrollToRect:WebCore::origin:WebCore::minimumScrollDistance:]):
(-[WKWebView _contentRectForUserInteraction]):
Use it in a bunch more places so we query the right view when zooming/scrolling for smart magnification.

* UIProcess/ios/WKPDFView.h:
* UIProcess/ios/WKPDFView.mm:
(-[WKPDFView scrollViewDidScroll:]):
(-[WKPDFView _revalidateViews]):
(-[WKPDFView zoom:to:atPoint:kind:]):
(-[WKPDFView resetZoom:]):
Implement zoom:to:atPoint:kind: and resetZoom: UIPDFPageViewDelegate methods.
Convert the rects/points and forward them on to the WKWebView to do the zoom.
resetZoom: doesn't provide the gesture origin, so we zoom out using the view center as our origin.
Avoid parenting new UIPDFPageViews while starting a zoom; they'll end up with a bizarre
animation on them and go flying across the screen (even before _isAnimatingZoom is set).

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@170515 268f45cc-cd09-0410-ab3c-d52691b4dbfc