DevTools: [CodeMirror] fix tab highlight

This patch fixes text editor "show whitespaces" feature to actually
switch off tab highlight when the feature gets disabled.

BUG=313616

Review URL: https://codereview.chromium.org/54763002

git-svn-id: svn://svn.chromium.org/blink/trunk@161065 bbb929c8-8fbe-4397-9dbb-9b2b20218538