DevTools: [Styles] Restore gray background for read-only rules

This regressed in r144925, which made all read-only rules have white background

TBR=vsevik

Review URL: https://codereview.chromium.org/131493002

git-svn-id: svn://svn.chromium.org/blink/trunk@164762 bbb929c8-8fbe-4397-9dbb-9b2b20218538