This is part 2 of adding support for --show-paint-rects in the compositor. The paint rect that is created by part 1 is plumbed through the appropriate places so that it can be visualized properly on the compositor HUD. Please note that this patch requires part 1 to be landed already.
Actually, this is already done now. On the webkit side the plumbing was done by https://bugs.webkit.org/show_bug.cgi?id=86255