Add pref to enable/disable palette tray.

ash/common/test/test_palette_delegate.cc

Index: ash/common/test/test_palette_delegate.cc
diff --git a/ash/common/test/test_palette_delegate.cc b/ash/common/test/test_palette_delegate.cc
index c9ea9d68fde8bf6d86fdb76da71746a10b54ddc1..0ca38204b25fad4fa7c583330d3d4af263e8c560 100644
--- a/ash/common/test/test_palette_delegate.cc
+++ b/ash/common/test/test_palette_delegate.cc
@@ -10,6 +10,12 @@ TestPaletteDelegate::TestPaletteDelegate() {}
 
 TestPaletteDelegate::~TestPaletteDelegate() {}
 
+std::unique_ptr<PaletteDelegate::EnableListenerSubscription>
+TestPaletteDelegate::AddPaletteEnableListener(
+    const EnableListener& on_state_changed) {
+  return nullptr;
+}
+
 void TestPaletteDelegate::CreateNote() {
   ++create_note_count_;
 }