DevTools: [CodeMirror] indent autodetection should fallback to default indent

This patch fixes indent autodetection to fallback to default indent
from user settings instead of "four spaces" value.

This patch additionally adds a change listener on the "autodetect indentation"
setting.

BUG=389109

Review URL: https://codereview.chromium.org/351213006

git-svn-id: svn://svn.chromium.org/blink/trunk@177103 bbb929c8-8fbe-4397-9dbb-9b2b20218538