Add pref to enable/disable palette tray.

ash/common/session/session_state_observer.cc

 // Copyright 2013 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/session/session_state_observer.h"
 
 #include "ash/common/session/session_state_delegate.h"
 #include "ash/common/wm_shell.h"
 
 namespace ash {
 
 ScopedSessionStateObserver::ScopedSessionStateObserver(
     SessionStateObserver* observer)
     : observer_(observer) {
   WmShell::Get()->GetSessionStateDelegate()->AddSessionStateObserver(observer_);
 }
 
 ScopedSessionStateObserver::~ScopedSessionStateObserver() {
-  WmShell::Get()->GetSessionStateDelegate()->RemoveSessionStateObserver(
-      observer_);
+  if (WmShell::Get()) {
+    WmShell::Get()->GetSessionStateDelegate()->RemoveSessionStateObserver(
+        observer_);
+  }
 }
 
 }  // namespace ash