ash uber tray: Fix a couple of crashes when quitting.

chrome/browser/chromeos/system/ash_system_tray_delegate.cc

Index: chrome/browser/chromeos/system/ash_system_tray_delegate.cc
diff --git a/chrome/browser/chromeos/system/ash_system_tray_delegate.cc b/chrome/browser/chromeos/system/ash_system_tray_delegate.cc
index 4c1a6f0958f05a5e8c6c6278c6f15056762289a8..2a80dbd1cf3bebd8a530c0b04db8a9da799106b2 100644
--- a/chrome/browser/chromeos/system/ash_system_tray_delegate.cc
+++ b/chrome/browser/chromeos/system/ash_system_tray_delegate.cc
@@ -32,7 +32,9 @@ class SystemTrayDelegate : public ash::SystemTrayDelegate,
   }
 
   virtual ~SystemTrayDelegate() {
-    AudioHandler::GetInstance()->RemoveVolumeObserver(this);
+    AudioHandler* audiohandler = AudioHandler::GetInstance();
+    if (audiohandler)
+      audiohandler->RemoveVolumeObserver(this);
     DBusThreadManager::Get()->GetPowerManagerClient()->RemoveObserver(this);
   }