DevTools: [CodeMirror] fix pretty-print behaviour

This patch fixes pretty print bug, which uses "AutoDetect" string
instead of indentation in case of auto-detect option switched on.

BUG=

Review URL: https://chromiumcodereview.appspot.com/19953003

git-svn-id: svn://svn.chromium.org/blink/trunk@154764 bbb929c8-8fbe-4397-9dbb-9b2b20218538