DevTools: allow injecting stylesheets as themes.

BUG=318566
R=vsevik@chromium.org

Review URL: https://codereview.chromium.org/91603003

git-svn-id: svn://svn.chromium.org/blink/trunk@162770 bbb929c8-8fbe-4397-9dbb-9b2b20218538