Close all windows when app shim quits.

apps/app_shim/app_shim_handler_mac.h

Index: apps/app_shim/app_shim_handler_mac.h
diff --git a/apps/app_shim/app_shim_handler_mac.h b/apps/app_shim/app_shim_handler_mac.h
index 507fe5f8410b055ea8f5cd26d3a3fcbb0ca6566d..e4a35fbd488adc9e3493bb672c270a2b52d4ee23 100644
--- a/apps/app_shim/app_shim_handler_mac.h
+++ b/apps/app_shim/app_shim_handler_mac.h
@@ -55,6 +55,9 @@ class AppShimHandler {
   // Invoked by the shim host when the shim process receives a focus event.
   virtual void OnShimFocus(Host* host) = 0;
 
+  // Invoked by the shim host when the shim process receives a quit event.
+  virtual void OnShimQuit(Host* host) = 0;
+
  protected:
   AppShimHandler() {}
   virtual ~AppShimHandler() {}