Implement "pinned" mode in ash.

ash/shell.h

Index: ash/shell.h
diff --git a/ash/shell.h b/ash/shell.h
index bb273232fa20c4616afcb859450cd5083a44f24e..56add433ee53376aa06013f222f88e73ef7dd959 100644
--- a/ash/shell.h
+++ b/ash/shell.h
@@ -172,6 +172,10 @@ namespace test {
 class ShellTestApi;
 }
 
+namespace wm {
+class PinnedController;
+}
+
 // Shell is a singleton object that presents the Shell API and implements the
 // RootWindow's delegate interface.
 //
@@ -588,6 +592,8 @@ class ASH_EXPORT Shell : public SystemModalContainerEventFilterDelegate,
   StickyKeysController* sticky_keys_controller() {
     return sticky_keys_controller_.get();
   }
+
+  wm::PinnedController* pinned_controller() { return pinned_controller_.get(); }
 #endif  // defined(OS_CHROMEOS)
 
   ToastManager* toast_manager() { return toast_manager_.get(); }
@@ -736,6 +742,7 @@ class ASH_EXPORT Shell : public SystemModalContainerEventFilterDelegate,
   std::unique_ptr<ui::UserActivityPowerManagerNotifier> user_activity_notifier_;
   std::unique_ptr<VideoActivityNotifier> video_activity_notifier_;
   std::unique_ptr<StickyKeysController> sticky_keys_controller_;
+  std::unique_ptr<wm::PinnedController> pinned_controller_;
   std::unique_ptr<ResolutionNotificationController>
       resolution_notification_controller_;
   std::unique_ptr<BluetoothNotificationController>