DevTools: [CodeMirror] restrict CodeMirror changes to rolls only

The patch adds presubmit check to enforce all changes to CodeMirror
folder to be rolls only.

The "roll" change is the one that has "roll CodeMirror" substring
in its description.

R=apavlov@chromium.org

Review URL: https://codereview.chromium.org/357443002

git-svn-id: svn://svn.chromium.org/blink/trunk@176845 bbb929c8-8fbe-4397-9dbb-9b2b20218538