Moves maintaining ShellObservers back to Shell

ash/common/system/chromeos/cast/tray_cast.cc

Index: ash/common/system/chromeos/cast/tray_cast.cc
diff --git a/ash/common/system/chromeos/cast/tray_cast.cc b/ash/common/system/chromeos/cast/tray_cast.cc
index 4ad2fd9fa44864f672eb98f0d7b6841bad0abd76..24d18c413ed4306b985fca6b024c291deb905697 100644
--- a/ash/common/system/chromeos/cast/tray_cast.cc
+++ b/ash/common/system/chromeos/cast/tray_cast.cc
@@ -29,6 +29,7 @@
 #include "ash/public/interfaces/cast_config.mojom.h"
 #include "ash/resources/grit/ash_resources.h"
 #include "ash/resources/vector_icons/vector_icons.h"
+#include "ash/shell.h"
 #include "ash/strings/grit/ash_strings.h"
 #include "base/bind.h"
 #include "base/strings/utf_string_conversions.h"
@@ -456,14 +457,14 @@ void CastDetailedView::HandleViewClicked(views::View* view) {
 
 TrayCast::TrayCast(SystemTray* system_tray)
     : SystemTrayItem(system_tray, UMA_CAST) {
-  WmShell::Get()->AddShellObserver(this);
+  Shell::GetInstance()->AddShellObserver(this);
   WmShell::Get()->cast_config()->AddObserver(this);
   WmShell::Get()->cast_config()->RequestDeviceRefresh();
 }
 
 TrayCast::~TrayCast() {
   WmShell::Get()->cast_config()->RemoveObserver(this);
-  WmShell::Get()->RemoveShellObserver(this);
+  Shell::GetInstance()->RemoveShellObserver(this);
 }
 
 void TrayCast::StartCastForTest(const std::string& receiver_id) {