Fix scrolling fixed objects containing abs pos objects

Need to recalculate the repaint rects for children inside fixed objects when we
find the repaint rects for fixed objects after scrolling so we invalidate the fixed
objects with the correct scroll position.

BUG=358550

Review URL: https://codereview.chromium.org/222473004

git-svn-id: svn://svn.chromium.org/blink/trunk@170700 bbb929c8-8fbe-4397-9dbb-9b2b20218538