DevTools: display paint rectangles on Layers panel 3d view

... toggled with the existing settings panel checkbox.

BUG=303130
R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/25666005

git-svn-id: svn://svn.chromium.org/blink/trunk@158802 bbb929c8-8fbe-4397-9dbb-9b2b20218538