Update repaint rects after Layout
    
Re-use LayoutState on the repaintTree walk of the rendering tree
to duplicate the fast-generation of repaint rects, and generate
the new repaint rect in this walk (instead of the layout walk) to
avoid having to worry about ancestors being moved around and two-
pass layout.

BUG=343896

Review URL: https://codereview.chromium.org/187983004

git-svn-id: svn://svn.chromium.org/blink/trunk@171083 bbb929c8-8fbe-4397-9dbb-9b2b20218538