Fix const correctness of containerForRepaint

Working on https://codereview.chromium.org/264183002/ I ran
into some const correctness issues with the fact that containerForRepaint
returns a non-const pointer, despite repaint being a const operation.
Fixing this before landing that patch.

BUG=320139

Review URL: https://codereview.chromium.org/276583002

git-svn-id: svn://svn.chromium.org/blink/trunk@173585 bbb929c8-8fbe-4397-9dbb-9b2b20218538