Implement "pinned" mode in ash.

ash/wm/workspace_controller.cc

Index: ash/wm/workspace_controller.cc
diff --git a/ash/wm/workspace_controller.cc b/ash/wm/workspace_controller.cc
index 3836a4bc38d00dcb706e0e0f05631cde34c5f9fb..e0c0be2ca095682539624db61a8d4bcb5bab525c 100644
--- a/ash/wm/workspace_controller.cc
+++ b/ash/wm/workspace_controller.cc
@@ -12,6 +12,7 @@
 #include "ash/common/wm/workspace/workspace_layout_manager.h"
 #include "ash/common/wm/workspace/workspace_layout_manager_backdrop_delegate.h"
 #include "ash/common/wm/workspace/workspace_layout_manager_delegate.h"
+#include "ash/common/wm_shell.h"
 #include "ash/root_window_controller.h"
 #include "ash/shelf/shelf_layout_manager.h"
 #include "ash/shell.h"
@@ -58,11 +59,13 @@ WorkspaceController::WorkspaceController(
   WmWindowAura::Get(viewport_)->SetLayoutManager(
       base::WrapUnique(layout_manager_));
   viewport_->AddPreTargetHandler(event_handler_.get());
+  WmShell::Get()->AddShellObserver(layout_manager_);
 }
 
 WorkspaceController::~WorkspaceController() {
-  viewport_->SetLayoutManager(NULL);
+  WmShell::Get()->RemoveShellObserver(layout_manager_);
   viewport_->RemovePreTargetHandler(event_handler_.get());
+  viewport_->SetLayoutManager(NULL);
 }
 
 wm::WorkspaceWindowState WorkspaceController::GetWindowState() const {