Cascade shutdown of instances

mash/browser_driver/browser_driver_application_delegate.cc

Index: mash/browser_driver/browser_driver_application_delegate.cc
diff --git a/mash/browser_driver/browser_driver_application_delegate.cc b/mash/browser_driver/browser_driver_application_delegate.cc
index 64ee8e4de7d7dbe05927247526514547f7c823a7..a2af11f931e32f8fc1769c9ecce524c17d45c5f8 100644
--- a/mash/browser_driver/browser_driver_application_delegate.cc
+++ b/mash/browser_driver/browser_driver_application_delegate.cc
@@ -7,6 +7,7 @@
 #include <stdint.h>
 
 #include "base/bind.h"
+#include "base/message_loop/message_loop.h"
 #include "components/mus/public/cpp/event_matcher.h"
 #include "mojo/shell/public/cpp/connection.h"
 #include "mojo/shell/public/cpp/connector.h"
@@ -63,6 +64,10 @@ bool BrowserDriverApplicationDelegate::AcceptConnection(
   return true;
 }
 
+void BrowserDriverApplicationDelegate::ShellConnectionLost() {
+  base::MessageLoop::current()->QuitWhenIdle();
+}
+
 void BrowserDriverApplicationDelegate::OnAccelerator(
     uint32_t id, mus::mojom::EventPtr event) {
   switch (static_cast<Accelerator>(id)) {