WIP: Eliminate event.target() usage from TrayEventFilter

ash/shell.cc

Index: ash/shell.cc
diff --git a/ash/shell.cc b/ash/shell.cc
index 8f4b51a1e00456341a85ffa62f2252a5dd4ceccd..db636ca027480e1a9af446bf60f79cb98826caf8 100644
--- a/ash/shell.cc
+++ b/ash/shell.cc
@@ -13,6 +13,7 @@
 #include "ash/accelerators/focus_manager_factory.h"
 #include "ash/ash_switches.h"
 #include "ash/autoclick/autoclick_controller.h"
+#include "ash/container_delegate.h"
 #include "ash/desktop_background/desktop_background_controller.h"
 #include "ash/desktop_background/desktop_background_view.h"
 #include "ash/desktop_background/user_wallpaper_delegate.h"
@@ -850,6 +851,7 @@ Shell::~Shell() {
   accessibility_delegate_.reset();
   new_window_delegate_.reset();
   media_delegate_.reset();
+  container_delegate_.reset();
   pointer_watcher_delegate_.reset();
 
   keyboard::KeyboardController::ResetInstance(nullptr);
@@ -1089,6 +1091,7 @@ void Shell::Init(const ShellInitParams& init_params) {
   accessibility_delegate_.reset(delegate_->CreateAccessibilityDelegate());
   new_window_delegate_.reset(delegate_->CreateNewWindowDelegate());
   media_delegate_.reset(delegate_->CreateMediaDelegate());
+  container_delegate_ = delegate_->CreateContainerDelegate();
   pointer_watcher_delegate_ = delegate_->CreatePointerWatcherDelegate();
 
   resize_shadow_controller_.reset(new ResizeShadowController());