Hide ash palette on lock screen, show it in guest mode.

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 5524ab5abc5a431015190ca70225784a8f8f852f..5a54f839f355d8b33098faf166b0351b4a18198a 100644
--- a/ash/common/system/chromeos/palette/palette_tray.h
+++ b/ash/common/system/chromeos/palette/palette_tray.h
@@ -45,6 +45,9 @@ class ASH_EXPORT PaletteTray : public TrayBackgroundView,
   // SessionStateObserver:
   void SessionStateChanged(SessionStateDelegate::SessionState state) override;
 
+  // ShellObserver:
+  void OnLockStateChanged(bool locked) override;
+
   // TrayBackgroundView:
   void ClickedOutsideBubble() override;
   base::string16 GetAccessibleNameForTray() override;