Implement "pinned" mode in ash.

ash/common/wm_shell_common.h

 // Copyright 2016 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.
 
 #ifndef ASH_COMMON_WM_SHELL_COMMON_H_
 #define ASH_COMMON_WM_SHELL_COMMON_H_
 
 #include <memory>
 
 #include "ash/ash_export.h"
@@ skipping 17 matching lines @@
   void CreateMruWindowTracker();
   void DeleteMruWindowTracker();
 
   // ShellObserver related functions:
   void AddShellObserver(ShellObserver* observer);
   void RemoveShellObserver(ShellObserver* observer);
   base::ObserverList<ShellObserver>* shell_observers() {
     return &shell_observers_;
   }
 
+  // Notifies |observers_| when entering or exiting pinned mode for
+  // |pinned_window|. Entering or exiting can be checked by looking at
+  // |pinned_window|'s window state.
   void NotifyPinnedStateChanged(WmWindow* pinned_window);
 
  private:
   std::unique_ptr<MruWindowTracker> mru_window_tracker_;
 
   base::ObserverList<ShellObserver> shell_observers_;
 
   DISALLOW_COPY_AND_ASSIGN(WmShellCommon);
 };
 
 }  // namespace ash
 
 #endif  // ASH_COMMON_WM_SHELL_COMMON_H_