Don't force full invalidation on box-sizing change

The renderer can already correctly handle invalidations of geometry changes
caused by box-sizing change.

BUG=375876
TEST=fast/repaint/box-sizing.html

Review URL: https://codereview.chromium.org/348493003

git-svn-id: svn://svn.chromium.org/blink/trunk@176716 bbb929c8-8fbe-4397-9dbb-9b2b20218538