DevTools: [CodeMirror] release text editor autocompletion experiment

This patch moves autocompletion from experiments into settings.

BUG=

Review URL: https://chromiumcodereview.appspot.com/21371003

git-svn-id: svn://svn.chromium.org/blink/trunk@155338 bbb929c8-8fbe-4397-9dbb-9b2b20218538