[iOS] Incremental updates during scroll-to-top shouldn’t count as “stable” updates

[iOS] Incremental updates during scroll-to-top shouldn’t count as “stable” updates
https://bugs.webkit.org/show_bug.cgi?id=132934

Reviewed by Simon Fraser.

* UIProcess/API/Cocoa/WKWebView.mm:
(-[WKWebView _updateVisibleContentRects]):
If we're in a scroll-to-top programmatic scroll, the update is not stable.
The last event comes in with _isScrollingToTop false, so we do our stable update.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@168895 268f45cc-cd09-0410-ab3c-d52691b4dbfc