Delete RenderLayer::updateLayerPositionsAfterDocumentScroll

This CL deletes RenderLayer::updateLayerPositionsAfterDocumentScroll and
unwinds all the complexity it created. Now that we're always passing
IsOverflowScroll, we can remove all the other flags for deciding when to clear
the clip cache. Also, now we always start the walk at RenderLayers that would
trigger HasSeenAncestorWithOverflowClip, so we can just assume that flag is
set.  That means there are no flags left!

Also, finally delete RenderLayerClipper::clearClipRects. Now everyone clears
all the descendant clip rects too.

Review URL: https://codereview.chromium.org/331653004

git-svn-id: svn://svn.chromium.org/blink/trunk@177144 bbb929c8-8fbe-4397-9dbb-9b2b20218538