ash: Add comment about where to add new accelerators

ash/accelerators/accelerator_controller_delegate_aura.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_ACCELERATORS_ACCELERATOR_HANDLER_AURA_H_
 #define ASH_ACCELERATORS_ACCELERATOR_HANDLER_AURA_H_
 
 #include <memory>
 
 #include "ash/common/accelerators/accelerator_controller_delegate.h"
 #include "base/macros.h"
 
 namespace ash {
 
 class ScreenshotDelegate;
 
 // Contains all the accelerators that have dependencies on portions of ash
-// not yet converted to ash/common.
+// not yet converted to ash/common. When adding a new accelerator that only
+// depends on ash/common code, add it to accelerator_controller.cc instead.
 class ASH_EXPORT AcceleratorControllerDelegateAura
     : public AcceleratorControllerDelegate {
  public:
   AcceleratorControllerDelegateAura();
   ~AcceleratorControllerDelegateAura() override;
 
   void SetScreenshotDelegate(
       std::unique_ptr<ScreenshotDelegate> screenshot_delegate);
   ScreenshotDelegate* screenshot_delegate() {
     return screenshot_delegate_.get();
@@ skipping 13 matching lines @@
 
  private:
   std::unique_ptr<ScreenshotDelegate> screenshot_delegate_;
 
   DISALLOW_COPY_AND_ASSIGN(AcceleratorControllerDelegateAura);
 };
 
 }  // namespace ash
 
 #endif  // ASH_ACCELERATORS_ACCELERATOR_HANDLER_AURA_H_