DevTools: Fix 'Disable Javascript' checkbox behavior.

BUG=347517
R=apavlov
NOTRY=true

Review URL: https://codereview.chromium.org/183883021

git-svn-id: svn://svn.chromium.org/blink/trunk@168739 bbb929c8-8fbe-4397-9dbb-9b2b20218538