Improve comments in RenderView::invalidatePaintForRectangle().

Also, Rename Repaint to InvalidatePaint in RenderView.

Review URL: https://codereview.chromium.org/424253003

git-svn-id: svn://svn.chromium.org/blink/trunk@179249 bbb929c8-8fbe-4397-9dbb-9b2b20218538