Implement "pinned" mode in ash.

ash/shell.h

Index: ash/shell.h
diff --git a/ash/shell.h b/ash/shell.h
index 159838aa9cb33288dcc0b7461d9ad5301d9280f3..65b3baadc26c67f843228d1cad89241a9aca5148 100644
--- a/ash/shell.h
+++ b/ash/shell.h
@@ -132,6 +132,7 @@ class ScopedTargetRootWindow;
 class ScreenAsh;
 class ScreenOrientationController;
 class ScreenshotController;
+class ScreenPinningController;
 class ScreenPositionController;
 class SessionStateDelegate;
 class Shelf;
@@ -570,8 +571,13 @@ class ASH_EXPORT Shell : public SystemModalContainerEventFilterDelegate,
   StickyKeysController* sticky_keys_controller() {
     return sticky_keys_controller_.get();
   }
+
 #endif  // defined(OS_CHROMEOS)
 
+  ScreenPinningController* screen_pinning_controller() {
+    return screen_pinning_controller_.get();
+  }
+
   ToastManager* toast_manager() { return toast_manager_.get(); }
 
   GPUSupport* gpu_support() { return gpu_support_.get(); }
@@ -711,6 +717,7 @@ class ASH_EXPORT Shell : public SystemModalContainerEventFilterDelegate,
       display_configuration_controller_;
 
   std::unique_ptr<LocaleNotificationController> locale_notification_controller_;
+  std::unique_ptr<ScreenPinningController> screen_pinning_controller_;
 
 #if defined(OS_CHROMEOS)
   std::unique_ptr<PowerEventObserver> power_event_observer_;