DevTools: [CSS] remove CSS Regions experiment from DevTools.

This patch removes CSS Regions from DevTools back-end and front-end
files.

BUG=341506

Review URL: https://codereview.chromium.org/163413005

git-svn-id: svn://svn.chromium.org/blink/trunk@167195 bbb929c8-8fbe-4397-9dbb-9b2b20218538