DevTools: [CodeMirror] add 'show whitespaces' setting change listener

This patch adds a listener on "show whitespaces" setting so that
this setting gets applied to the editors without devtools reload.

BUG=

Review URL: https://chromiumcodereview.appspot.com/18440004

git-svn-id: svn://svn.chromium.org/blink/trunk@153431 bbb929c8-8fbe-4397-9dbb-9b2b20218538