Add WorkspaceObserver to observe changes in workspace state

ui/aura_shell/shell.cc

 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ui/aura_shell/shell.h"
 
 #include "base/bind.h"
 #include "ui/aura/desktop.h"
 #include "ui/aura/screen_aura.h"
 #include "ui/aura/toplevel_window_container.h"
 #include "ui/aura/window.h"
 #include "ui/aura/window_types.h"
 #include "ui/aura_shell/default_container_event_filter.h"
+#include "ui/aura_shell/default_container_layout_manager.h"
 #include "ui/aura_shell/desktop_layout_manager.h"
 #include "ui/aura_shell/launcher/launcher.h"
 #include "ui/aura_shell/shell_delegate.h"
 #include "ui/aura_shell/shell_factory.h"
 #include "ui/aura_shell/shell_window_ids.h"
 #include "ui/aura_shell/workspace/workspace_controller.h"
 #include "ui/gfx/compositor/layer.h"
 #include "ui/gfx/compositor/layer_animator.h"
 #include "views/widget/native_widget_aura.h"
 #include "views/widget/widget.h"
@@ skipping 96 matching lines @@
   desktop_layout->set_status_area_widget(internal::CreateStatusArea());
 
   desktop_window->screen()->set_work_area_insets(
       gfx::Insets(
           0, kWorkAreaHorizontalMargin,
           launcher_->widget()->GetWindowScreenBounds().height(),
           kWorkAreaHorizontalMargin));
 
   workspace_controller_.reset(
       new internal::WorkspaceController(toplevel_container));
+  toplevel_container->SetLayoutManager(
+      new internal::DefaultContainerLayoutManager(
+          workspace_controller_->workspace_manager()));
 
   // Force a layout.
   desktop_layout->OnWindowResized();
 }
 
 void Shell::SetDelegate(ShellDelegate* delegate) {
   delegate_.reset(delegate);
 }
 
 aura::Window* Shell::GetContainer(int container_id) {
@@ skipping 32 matching lines @@
 }
 
 aura::Window* Shell::GetTopmostWindowToActivate(aura::Window* ignore) const {
   const aura::ToplevelWindowContainer* container =
       GetContainer(internal::kShellWindowId_DefaultContainer)->
           AsToplevelWindowContainer();
   return container->GetTopmostWindowToActivate(ignore);
 }
 
 }  // namespace aura_shell