[DevTools] CM Editor: fix gutter/line numbers colors.

Currently CM gutter colors do not correspond non-CM style.
(See ctrl-shift-f results.)

Review URL: https://chromiumcodereview.appspot.com/18991009

git-svn-id: svn://svn.chromium.org/blink/trunk@154106 bbb929c8-8fbe-4397-9dbb-9b2b20218538