[iOS WK2] position:fixed inside accelerated overflow:scroll is jumpy
https://bugs.webkit.org/show_bug.cgi?id=134426
<rdar://problem/17474523>

Reviewed by Tim Horton.

After committing a new layer tree (with possibly stale position:fixed layer
positions), we need the scrolling tree to update those positions based on
the current scroll offset.

Source/WebCore:
Give ScrollingTreeScrollingNode an implementation of updateLayersAfterAncestorChange()
which is required to update fixed/sticky child nodes.

* WebCore.exp.in:
* page/scrolling/ScrollingTreeScrollingNode.cpp:
(WebCore::ScrollingTreeScrollingNode::updateLayersAfterAncestorChange):
* page/scrolling/ScrollingTreeScrollingNode.h:

Source/WebKit2:
To achieve that, implement ScrollingTreeOverflowScrollingNodeIOS::updateLayersAfterAncestorChange()
and have it add to the cumulative delta the difference between the last committed scroll
position and the current scroll position.

Also make sure that ScrollingTreeOverflowScrollingNodeIOS doesn't call back to scrollViewDidScroll()
when we're updating its scroll position inside a scrolling tree commit.

* UIProcess/Scrolling/ios/ScrollingTreeOverflowScrollingNodeIOS.h:
* UIProcess/Scrolling/ios/ScrollingTreeOverflowScrollingNodeIOS.mm:
(WebKit::ScrollingTreeOverflowScrollingNodeIOS::ScrollingTreeOverflowScrollingNodeIOS):
(WebKit::ScrollingTreeOverflowScrollingNodeIOS::updateAfterChildren):
(WebKit::ScrollingTreeOverflowScrollingNodeIOS::updateLayersAfterAncestorChange):
(WebKit::ScrollingTreeOverflowScrollingNodeIOS::scrollViewDidScroll):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@170568 268f45cc-cd09-0410-ab3c-d52691b4dbfc