Split out repaint code from RenderBlockFlow layout.

Currently the RenderBlockFlow layout code has a FIXME to extract some repaint
logic into a helper function. This extraction is needed for the repaintAfterLayout
work because we need to be able to call it when layout is complete.

This CL pulls the code out into a method defined in RenderObject and overridden
in RenderBlockFlow.

BUG=310398

Review URL: https://codereview.chromium.org/110543002

git-svn-id: svn://svn.chromium.org/blink/trunk@163460 bbb929c8-8fbe-4397-9dbb-9b2b20218538