DevTools: [CodeMirror] roll codemirror to @687fb51

This patch rolls CodeMirror to @687fb51. The reason for that is
different handling of global events such as window.onResize.

BUG=371218
TBR=pfeldman, vsevik

Review URL: https://codereview.chromium.org/273763003

git-svn-id: svn://svn.chromium.org/blink/trunk@173960 bbb929c8-8fbe-4397-9dbb-9b2b20218538