rubber band
https://bugs.webkit.org/show_bug.cgi?id=124260

Reviewed by Tim Horton.

This patch adds a new enum called ScrollBehaviorForFixedElements, which has
two values, StickToDocumentBounds or StickToWindowBounds. StickToDocumentBounds 
corresponds to our current behavior, where fixed elements constrain 
their positions during a rubber-band so that they stay stuck to the document.
The new mode, StickToWindowBounds, will cause fixed elements to always stay
fixed relative to the window.

scrollOffsetForFixedPosition() now takes a new parameter for the fixed behavior
so that it knows whether or not to constrain the position.
* page/FrameView.cpp:
(WebCore::FrameView::scrollOffsetForFixedPosition):
        
Right now, just return StickToDocumentBounds and retain existing behavior.
(WebCore::FrameView::scrollBehaviorForFixedElements):
* page/FrameView.h:

The scrolling thread needs to know about the fixed element scroll behavior,
so this code makes ScrollingStateScrollingNodes keep track of that 
information to pass over to the scrolling thread.
* page/scrolling/ScrollingStateScrollingNode.cpp:
(WebCore::ScrollingStateScrollingNode::ScrollingStateScrollingNode):
(WebCore::ScrollingStateScrollingNode::setScrollBehaviorForFixedElements):
* page/scrolling/ScrollingStateScrollingNode.h:
* page/scrolling/ScrollingTreeScrollingNode.cpp:
(WebCore::ScrollingTreeScrollingNode::ScrollingTreeScrollingNode):
(WebCore::ScrollingTreeScrollingNode::updateBeforeChildren):
* page/scrolling/ScrollingTreeScrollingNode.h:
(WebCore::ScrollingTreeScrollingNode::scrollBehaviorForFixedElements):
* page/scrolling/mac/ScrollingCoordinatorMac.h:
* page/scrolling/mac/ScrollingCoordinatorMac.mm:
(WebCore::ScrollingCoordinatorMac::frameViewRootLayerDidChange):
(WebCore::ScrollingCoordinatorMac::setScrollBehaviorForFixedElementsForNode):
* page/scrolling/mac/ScrollingTreeScrollingNodeMac.mm:
(WebCore::ScrollingTreeScrollingNodeMac::setScrollLayerPosition):
        
Here is the new enum.
* platform/ScrollTypes.h:
        
Another place where we only want to constrain scroll position if that is the
mode we are in.
* rendering/RenderLayerCompositor.cpp:
(WebCore::RenderLayerCompositor::customPositionForVisibleRectComputation):



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@159300 268f45cc-cd09-0410-ab3c-d52691b4dbfc