DevTools: Hide Overrides note when a node or quad is highlighted.

This prevents users from seeing the entire box model rectangles and element tooltip.

R=pfeldman, vsevik
BUG=302862

Review URL: https://codereview.chromium.org/25706002

git-svn-id: svn://svn.chromium.org/blink/trunk@158804 bbb929c8-8fbe-4397-9dbb-9b2b20218538