kiosk: Network connectivity test during launch.

- Add NetworkStateInformer and ErrorScreenActor to
  AppLaunchSplashScreenHandler;
- AppLaunchSplashScreenHandler sends network state
  to its delegate (AppLaunchController) during network
  check;
- AppLaunchController decides when to show network
  configure UI and call AppLaunchSplashScreenActor
  to show it (via ErrorScreenActor);
- Show network config UI for enterprise kiosk;
- Add a 'Reboot' button for kiosk network error;
- Add a PromptForNetworkWhenOffline boolean field to
  DeviceLocalAccountsProto to control whether to show
  network config UI for enterprise managed kiosk;

BUG=314710
TEST=KioskTest.LaunchAppNetworkDown/Portal

Review URL: https://codereview.chromium.org/79113002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238873 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238873
Expose swapping the web contents out to observers

BUG=321901

Review URL: https://codereview.chromium.org/89563002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238874 0039d316-1c4b-4281-b951-d872f2087c98
Add null check for SigninManagerFactory::GetForProfile(profile)

BUG=324862

Review URL: https://codereview.chromium.org/102033003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238875 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238875