StickyPositionConstraints should store the constrainingRectAtLastLayout
https://bugs.webkit.org/show_bug.cgi?id=118999

Reviewed by Simon Fraser.

Much like how FixedPositionConstraints store a viewportRectAtLastLayout, 
StickyConstraints should store a constrainingRectAtLastLayout. We'll need this to 
get sticky right in overflow areas once overflow areas scroll on the scrolling 
thread. 

* page/scrolling/ScrollingConstraints.h:
(WebCore::StickyPositionViewportConstraints::StickyPositionViewportConstraints):
(WebCore::StickyPositionViewportConstraints::constrainingRectAtLastLayout):
(WebCore::StickyPositionViewportConstraints::setConstrainingRectAtLastLayout):
* rendering/RenderBoxModelObject.cpp:
(WebCore::RenderBoxModelObject::computeStickyPositionConstraints):



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@153032 268f45cc-cd09-0410-ab3c-d52691b4dbfc