DevTools: Persist edited stylesheets content when css agent is disabled

We need to maintain agent state to correctly restore it after reattaching.

BUG=385011

Review URL: https://codereview.chromium.org/334623007

git-svn-id: svn://svn.chromium.org/blink/trunk@176631 bbb929c8-8fbe-4397-9dbb-9b2b20218538