DevTools: [regression r161185] unable to disable userAgent, css and sensors emulation
includes fix for nasty stall while navigating back to webui
includes disabling controls while remote debugging

BUG=314555

Review URL: https://codereview.chromium.org/57693002

git-svn-id: svn://svn.chromium.org/blink/trunk@161244 bbb929c8-8fbe-4397-9dbb-9b2b20218538