Remove "solo window" feature

ash/root_window_controller.cc

Index: ash/root_window_controller.cc
diff --git a/ash/root_window_controller.cc b/ash/root_window_controller.cc
index 1af782d079b4a04fcfb9e8af165b6ded86a2a3d9..fcc7f26179df7b6c604b9a00e39b6ed7f513e0ee 100644
--- a/ash/root_window_controller.cc
+++ b/ash/root_window_controller.cc
@@ -36,7 +36,6 @@
 #include "ash/wm/panels/panel_window_event_handler.h"
 #include "ash/wm/root_window_layout_manager.h"
 #include "ash/wm/screen_dimmer.h"
-#include "ash/wm/solo_window_tracker.h"
 #include "ash/wm/stacking_controller.h"
 #include "ash/wm/status_area_layout_manager.h"
 #include "ash/wm/system_background_controller.h"
@@ -452,13 +451,6 @@ void RootWindowController::OnWallpaperAnimationFinished(views::Widget* widget) {
 void RootWindowController::CloseChildWindows() {
   mouse_event_target_.reset();
 
-  // |solo_window_tracker_| must be shut down before windows are destroyed.
-  if (solo_window_tracker_) {
-    if (docked_layout_manager_)
-      docked_layout_manager_->RemoveObserver(solo_window_tracker_.get());
-    solo_window_tracker_.reset();
-  }
-
   // Deactivate keyboard container before closing child windows and shutting
   // down associated layout managers.
   DeactivateKeyboard(Shell::GetInstance()->keyboard_controller());
@@ -694,10 +686,6 @@ void RootWindowController::Init(RootWindowType root_window_type,
     if (shell->session_state_delegate()->NumberOfLoggedInUsers())
       shelf()->CreateShelf();
   }
-
-  solo_window_tracker_.reset(new SoloWindowTracker(root_window_.get()));
-  if (docked_layout_manager_)
-    docked_layout_manager_->AddObserver(solo_window_tracker_.get());
 }
 
 void RootWindowController::InitLayoutManagers() {