Moves MruWindowTracker to WmShell

ash/mus/bridge/wm_shell_mus.cc

Index: ash/mus/bridge/wm_shell_mus.cc
diff --git a/ash/mus/bridge/wm_shell_mus.cc b/ash/mus/bridge/wm_shell_mus.cc
index effc0ec58b469c140938b8fbf46d539c35b902a7..829e4a921a1b50fe1a8dd392f35478a174a720e6 100644
--- a/ash/mus/bridge/wm_shell_mus.cc
+++ b/ash/mus/bridge/wm_shell_mus.cc
@@ -89,9 +89,12 @@ WmShellMus::WmShellMus(::mus::WindowTreeClient* client)
     : client_(client), session_state_delegate_(new SessionStateDelegateStub) {
   client_->AddObserver(this);
   WmShell::Set(this);
+
+  CreateMruWindowTracker();
 }
 
 WmShellMus::~WmShellMus() {
+  DeleteMruWindowTracker();
   RemoveClientObserver();
   WmShell::Set(nullptr);
 }
@@ -160,16 +163,6 @@ WmWindow* WmShellMus::GetRootWindowForNewWindows() {
   return root_window_controllers_[0]->GetWindow();
 }
 
-std::vector<WmWindow*> WmShellMus::GetMruWindowList() {
-  NOTIMPLEMENTED();
-  return std::vector<WmWindow*>();
-}
-
-std::vector<WmWindow*> WmShellMus::GetMruWindowListIgnoreModals() {
-  NOTIMPLEMENTED();
-  return std::vector<WmWindow*>();
-}
-
 bool WmShellMus::IsForceMaximizeOnFirstRun() {
   NOTIMPLEMENTED();
   return false;