DevTools: [CodeMirror] roll CodeMirror to version 4.1, rev@708f68c

This roll fixes couple of bugs and supports chromium overlay scrollbars.

TBR=vsevik, pfeldman
BUG=359068

Review URL: https://codereview.chromium.org/249223002

git-svn-id: svn://svn.chromium.org/blink/trunk@172364 bbb929c8-8fbe-4397-9dbb-9b2b20218538