Add pref to enable/disable palette tray.

ash/shell/shell_delegate_impl.cc

 // Copyright (c) 2012 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.
 
 #include "ash/shell/shell_delegate_impl.h"
 
 #include "ash/app_list/app_list_presenter_delegate_factory.h"
 #include "ash/common/accessibility_delegate.h"
 #include "ash/common/default_accessibility_delegate.h"
 #include "ash/common/gpu_support_stub.h"
@@ skipping 64 matching lines @@
   MediaCaptureState GetMediaCaptureState(UserIndex index) override {
     return MEDIA_CAPTURE_VIDEO;
   }
 
  private:
   DISALLOW_COPY_AND_ASSIGN(MediaDelegateImpl);
 };
 
 class PaletteDelegateImpl : public PaletteDelegate {
  public:
-  PaletteDelegateImpl() {};
-  ~PaletteDelegateImpl() override {};
+  PaletteDelegateImpl() {}
+  ~PaletteDelegateImpl() override {}
+
+  // PaletteDelegate:
+  std::unique_ptr<EnableListenerSubscription> AddPaletteEnableListener(
+      const EnableListener& on_state_changed) override {
+    on_state_changed.Run(false);
+    return std::unique_ptr<EnableListenerSubscription>();

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

return nullptr?

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

Done.

+  }
 
  private:
   DISALLOW_COPY_AND_ASSIGN(PaletteDelegateImpl);
 };
 
 class SessionStateDelegateImpl : public SessionStateDelegate {
  public:
   SessionStateDelegateImpl()
       : screen_locked_(false), user_info_(new user_manager::UserInfoImpl()) {}
 
@@ skipping 178 matching lines @@
 base::string16 ShellDelegateImpl::GetProductName() const {
   return base::string16();
 }
 
 gfx::Image ShellDelegateImpl::GetDeprecatedAcceleratorImage() const {
   return gfx::Image();
 }
 
 }  // namespace shell
 }  // namespace ash