Move the repainting logic out of RenderLayer

This change moves the logic for repainting (repaint rects, ...)
out of RenderLayer. Unfortunately a lot of classes rely on it so
the new object had to be exposed throughout core/.

BUG=260898

Review URL: https://codereview.chromium.org/25492005

git-svn-id: svn://svn.chromium.org/blink/trunk@158728 bbb929c8-8fbe-4397-9dbb-9b2b20218538