Add pref to enable/disable palette tray.

ash/common/test/test_palette_delegate.cc

 // 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.
 
 #include "ash/common/test/test_palette_delegate.h"
 
 namespace ash {
 
 TestPaletteDelegate::TestPaletteDelegate() {}
 
 TestPaletteDelegate::~TestPaletteDelegate() {}
 
+std::unique_ptr<PaletteDelegate::EnableListenerSubscription>
+TestPaletteDelegate::AddPaletteEnableListener(
+    const EnableListener& on_state_changed) {
+  return nullptr;
+}
+
 void TestPaletteDelegate::CreateNote() {
   ++create_note_count_;
 }
 
 bool TestPaletteDelegate::HasNoteApp() {
   ++has_note_app_count_;
   return has_note_app_;
 }
 
 void TestPaletteDelegate::SetPartialMagnifierState(bool enabled) {
@@ skipping 10 matching lines @@
 
 void TestPaletteDelegate::TakeScreenshot() {
   ++take_screenshot_count_;
 }
 
 void TestPaletteDelegate::TakePartialScreenshot() {
   ++take_partial_screenshot_count_;
 }
 
 }  // namespace ash