DevTools: Provide indication of the target tab inactivity in screencast view (Blink side)

BUG=24242004

Review URL: https://chromiumcodereview.appspot.com/24145005

git-svn-id: svn://svn.chromium.org/blink/trunk@158117 bbb929c8-8fbe-4397-9dbb-9b2b20218538