Fix repaint rects of children of sub-pixel composited elements

Blink r164561 fixed a longstanding issue with sub-pixel shift of composited
elements, but failed to correctly adjust repaint rects of child by the newly added
sub-pixel offset. This patch moves the adjustment down to the composited
path of RenderLayerRepainter::setBackingNeedsRepaintInRect.

BUG=334588

Review URL: https://codereview.chromium.org/136723011

git-svn-id: svn://svn.chromium.org/blink/trunk@165584 bbb929c8-8fbe-4397-9dbb-9b2b20218538