Forcibly terminate the metro viewer

On trybots, the viewer shutdown is frequently taking 16s, making ash_unittests
as a whole take ~20m. Killing the viewer more aggressively makes all the tests
run at the "normal" time in the 1-500ms range, dropping total time to 5-6m.

R=cpu@chromium.org
BUG=411147

Review URL: https://codereview.chromium.org/584513004

Cr-Commit-Position: refs/heads/master@{#295749}

win8/metro_driver/chrome_app_view_ash.cc

@@ -970,6 +970,9 @@ void ChromeAppViewAsh::OnMetroExit(MetroTerminateMethod method) {
 
     globals.app_exit->Exit();
   }
+
+  // Try really hard, see http://crbug.com/411147 for details.
+  ::TerminateProcess(::GetCurrentProcess(), 0);
 }
 
 void ChromeAppViewAsh::OnInputSourceChanged() {