WIP: Eliminate event.target() usage from TrayEventFilter

ash/mus/shell_delegate_mus.cc

Index: ash/mus/shell_delegate_mus.cc
diff --git a/ash/mus/shell_delegate_mus.cc b/ash/mus/shell_delegate_mus.cc
index 654c9cc109c79b31137b4a90abc58ea72a359cba..eeefb2b044c77b50d759cf574ba4684c2d3c7210 100644
--- a/ash/mus/shell_delegate_mus.cc
+++ b/ash/mus/shell_delegate_mus.cc
@@ -9,6 +9,7 @@
 #include "ash/gpu_support_stub.h"
 #include "ash/media_delegate.h"
 #include "ash/mus/app_list_presenter_mus.h"
+#include "ash/mus/container_delegate_mus.h"
 #include "ash/mus/context_menu_mus.h"
 #include "ash/mus/pointer_watcher_delegate_mus.h"
 #include "ash/mus/shelf_delegate_mus.h"
@@ -208,6 +209,11 @@ MediaDelegate* ShellDelegateMus::CreateMediaDelegate() {
   return new MediaDelegateStub;
 }
 
+std::unique_ptr<ContainerDelegate>
+ShellDelegateMus::CreateContainerDelegate() {
+  return base::WrapUnique(new ContainerDelegateMus);
+}
+
 std::unique_ptr<PointerWatcherDelegate>
 ShellDelegateMus::CreatePointerWatcherDelegate() {
   return base::WrapUnique(new PointerWatcherDelegateMus);