ash: Add comment about where to add new accelerators

ash/accelerators/accelerator_controller_delegate_aura.h

Index: ash/accelerators/accelerator_controller_delegate_aura.h
diff --git a/ash/accelerators/accelerator_controller_delegate_aura.h b/ash/accelerators/accelerator_controller_delegate_aura.h
index 7d98ba09baf42479cf6e31210b28ed8644b8a2cf..3f683820f7d90c489a17998c583ebe5a6a480bbb 100644
--- a/ash/accelerators/accelerator_controller_delegate_aura.h
+++ b/ash/accelerators/accelerator_controller_delegate_aura.h
@@ -15,7 +15,8 @@ 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: