DevTools: [CodeMirror] roll codemirror presubmit instructions

This patch adds link to CodeMirror roll instructions in the
error message of CodeMirror presubmit check.

R=apavlov, pfeldman

Review URL: https://codereview.chromium.org/352143003

git-svn-id: svn://svn.chromium.org/blink/trunk@176935 bbb929c8-8fbe-4397-9dbb-9b2b20218538