Implement "pinned" mode in ash.

ash/common/wm/always_on_top_controller.cc

Index: ash/common/wm/always_on_top_controller.cc
diff --git a/ash/common/wm/always_on_top_controller.cc b/ash/common/wm/always_on_top_controller.cc
index 275a4462df10cca3ace08399117dbfd1645a7ffc..92ff27d47122342f7dbd82baad7c2a51a84482a2 100644
--- a/ash/common/wm/always_on_top_controller.cc
+++ b/ash/common/wm/always_on_top_controller.cc
@@ -7,6 +7,7 @@
 #include "ash/common/shell_window_ids.h"
 #include "ash/common/wm/workspace/workspace_layout_manager.h"
 #include "ash/common/wm/workspace/workspace_layout_manager_delegate.h"
+#include "ash/common/wm_shell.h"
 #include "ash/common/wm_window.h"
 #include "ash/common/wm_window_property.h"
 #include "base/memory/ptr_util.h"
@@ -17,14 +18,17 @@ AlwaysOnTopController::AlwaysOnTopController(WmWindow* viewport)
     : always_on_top_container_(viewport) {
   always_on_top_container_->SetLayoutManager(
       base::WrapUnique(new WorkspaceLayoutManager(viewport, nullptr)));
+  WmShell::Get()->AddShellObserver(GetLayoutManager());
   // Container should be empty.
   DCHECK(always_on_top_container_->GetChildren().empty());
   always_on_top_container_->AddObserver(this);
 }
 
 AlwaysOnTopController::~AlwaysOnTopController() {
-  if (always_on_top_container_)
+  if (always_on_top_container_) {
     always_on_top_container_->RemoveObserver(this);
+    WmShell::Get()->RemoveShellObserver(GetLayoutManager());
+  }
 }
 
 WmWindow* AlwaysOnTopController::GetContainer(WmWindow* window) const {
@@ -43,7 +47,9 @@ WorkspaceLayoutManager* AlwaysOnTopController::GetLayoutManager() const {
 
 void AlwaysOnTopController::SetLayoutManagerForTest(
     std::unique_ptr<WorkspaceLayoutManager> layout_manager) {
+  WmShell::Get()->RemoveShellObserver(GetLayoutManager());
   always_on_top_container_->SetLayoutManager(std::move(layout_manager));
+  WmShell::Get()->AddShellObserver(GetLayoutManager());
 }
 
 void AlwaysOnTopController::OnWindowTreeChanged(
@@ -70,6 +76,7 @@ void AlwaysOnTopController::OnWindowPropertyChanged(WmWindow* window,
 void AlwaysOnTopController::OnWindowDestroying(WmWindow* window) {
   if (window == always_on_top_container_) {
     always_on_top_container_->RemoveObserver(this);
+    WmShell::Get()->RemoveShellObserver(GetLayoutManager());
     always_on_top_container_ = nullptr;
   }
 }