Delete unused RenderLayer::CheckForPaintInvalidation

No one reads this flag anymore, we can remove it.

R=chrishtr@chromium.org

Review URL: https://codereview.chromium.org/425323002

git-svn-id: svn://svn.chromium.org/blink/trunk@179246 bbb929c8-8fbe-4397-9dbb-9b2b20218538