New avatar menu: Don't show the "Sign in to Chrome" button if sign-in is not allowed.
If the SigninManager won't allow signing in, then showing the button makes no sense. The hotdog menu does much the same thing (https://code.google.com/p/chromium/codesearch#chromium/src/chrome/browser/ui/toolbar/wrench_menu_model.cc&l=583)
Mac implementation to follow.
BUG=372812,310640
Review URL: https://codereview.chromium.org/
291843002
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@272019 0039d316-1c4b-4281-b951-d872f2087c98