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

BUG=294840

Review URL: https://chromiumcodereview.appspot.com/24242004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@224642 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 224642