This patch rolls CodeMirror and all its plugins to v3.20. The new
version of css mode changed its internal name from 'css-base' to
just 'css', which implied changes in CodeMirrorUtils.js file.

NB: headlesscodemirror.js if not taken from v3.20 distribution as it has bugs; it's fixed version could be
found here: https://github.com/aslushnikov/CodeMirror/blob/925130e8b455c137b385c611a67dcb7d582c0281/addon/runmode/runmode-standalone.js

R=apavlov@chromium.org

Review URL: https://codereview.chromium.org/136433002

git-svn-id: svn://svn.chromium.org/blink/trunk@164974 bbb929c8-8fbe-4397-9dbb-9b2b20218538