ash: Add comment about where to add new accelerators

ash/accelerators/accelerator_controller_delegate_aura.cc

Index: ash/accelerators/accelerator_controller_delegate_aura.cc
diff --git a/ash/accelerators/accelerator_controller_delegate_aura.cc b/ash/accelerators/accelerator_controller_delegate_aura.cc
index 654edd57b44a4a8ebadda05b631ca00f2ec11d44..efc09f6610e5eef2fee77d3bfe6678f9574515a1 100644
--- a/ash/accelerators/accelerator_controller_delegate_aura.cc
+++ b/ash/accelerators/accelerator_controller_delegate_aura.cc
@@ -361,6 +361,8 @@ void AcceleratorControllerDelegateAura::SetScreenshotDelegate(
 
 bool AcceleratorControllerDelegateAura::HandlesAction(
     AcceleratorAction action) {
+  // NOTE: When adding a new accelerator that only depends on //ash/common code,
+  // add it to accelerator_controller.cc instead. See class comment.
   switch (action) {
     case DEBUG_TOGGLE_DESKTOP_BACKGROUND_MODE:
     case DEBUG_TOGGLE_DEVICE_SCALE_FACTOR: