Add pref to enable/disable palette tray.

ash/common/palette_delegate.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_PALETTE_DELEGATE_H_
 #define ASH_COMMON_PALETTE_DELEGATE_H_
 
 #include "ash/ash_export.h"
+#include "base/callback.h"
+#include "base/callback_list.h"
 #include "base/macros.h"
 
 namespace ash {
 
 // This delegate allows the UI code in ash, e.g. |PaletteTray|, to perform
 // Chrome-specific actions.
 class PaletteDelegate {
  public:
+  using EnableListener = base::Callback<void(bool)>;
+  using EnableListenerSubscription =
+      base::CallbackList<void(bool)>::Subscription;
+
   virtual ~PaletteDelegate() {}
 
-  // TODO(jdufault): This delegate will be implemented in later CLs.
+  // Sets callback function that will receive the current state of the palette
+  // enabled pref. The callback will be invoked once the initial pref value is
+  // available.
+  virtual std::unique_ptr<EnableListenerSubscription> AddPaletteEnableListener(
+      const EnableListener& on_state_changed) = 0;
 
  private:
   DISALLOW_ASSIGN(PaletteDelegate);
 };
 
 }  // namespace ash
 
 #endif  // ASH_COMMON_PALETTE_DELEGATE_H_