Add pref to enable/disable palette tray.

ash/common/system/chromeos/palette/palette_tray.h

Index: ash/common/system/chromeos/palette/palette_tray.h
diff --git a/ash/common/system/chromeos/palette/palette_tray.h b/ash/common/system/chromeos/palette/palette_tray.h
index 18d900ab1bc31f3f6a8337343b9d0a6aa7f2578b..58f488a625b2b3d8e47c3284e50392ac545fce1d 100644
--- a/ash/common/system/chromeos/palette/palette_tray.h
+++ b/ash/common/system/chromeos/palette/palette_tray.h
@@ -9,11 +9,13 @@
 #include <memory>
 
 #include "ash/ash_export.h"
+#include "ash/common/palette_delegate.h"
 #include "ash/common/session/session_state_observer.h"
 #include "ash/common/shell_observer.h"
 #include "ash/common/system/chromeos/palette/palette_tool_manager.h"
 #include "ash/common/system/tray/tray_background_view.h"
 #include "base/macros.h"
+#include "base/memory/weak_ptr.h"
 
 namespace views {
 class ImageView;
@@ -82,15 +84,24 @@ class ASH_EXPORT PaletteTray : public TrayBackgroundView,
   // Sets the icon to visible if the palette can be used.
   void UpdateIconVisibility();
 
+  // Called when the palette enabled pref has changed.
+  void OnPaletteEnabledPrefChanged(bool enabled);
+
   bool OpenBubble();
   void AddToolsToView(views::View* host);
 
   std::unique_ptr<PaletteToolManager> palette_tool_manager_;
   std::unique_ptr<TrayBubbleWrapper> bubble_;
 
+  // Manages the callback OnPaletteEnabledPrefChanged callback registered to
+  // the PaletteDelegate instance.
+  std::unique_ptr<PaletteDelegate::EnableListenerSubscription> palette_enabled_;

[stevenjb, 2016/08/23 17:36:41]

This is a confusing variable name, I would use palette_enabled_subscription_ or
somesuch.

[jdufault, 2016/08/23 22:49:56]

Done.

+
   // Weak pointer, will be parented by TrayContainer for its lifetime.
   views::ImageView* icon_;
 
+  base::WeakPtrFactory<PaletteTray> weak_factory_;
+
   DISALLOW_COPY_AND_ASSIGN(PaletteTray);
 };