Make MessageCenter a singleton object and build on Windows.

ui/message_center/message_center.cc

 // Copyright (c) 2012 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 "ui/message_center/message_center.h"
 
 #include "base/logging.h"
+#include "base/memory/singleton.h"
 
 namespace message_center {
 
 //------------------------------------------------------------------------------
 
-MessageCenter::MessageCenter(Host* host)
-    : host_(host),
-      delegate_(NULL) {
-  notification_list_.reset(new NotificationList(this));
+// static
+MessageCenter* MessageCenter::GetInstance() {
+  return Singleton<MessageCenter>::get();
 }
 
 MessageCenter::~MessageCenter() {
   notification_list_.reset();
 }
 
+void MessageCenter::SetHost(Host* host) {

[stevenjb, 2012/12/27 19:49:34]

DCHECK(!host_) since we don't support multiple hosts or host handoff.

[dewittj, 2012/12/27 23:38:50]

Since the MessageCenter is now not being destroyed until exit (and separately
from the host), I figured it'd be best to be able to SetHost(NULL) when I
destroy the host to prevent calling OnMessageCenterChanged on a destroyed
object.

Unfortunately, the DCHECK doesn't seem to work with the ash_unittests which
create multiple status trays without exiting the process.  Is there a reason we
don't support host handoff?  Since the only thing the host is doing is receiving
events it doesn't seem like handoff is a big deal.

If that DCHECK is important, I'll have to rethink the Singleton<> paradigm and
use something else.

[stevenjb, 2013/01/02 16:35:02]

Thinking about this some more, it seems like we need to support multiple "hosts"
since we may have multiple status trays "hosting" the message center when there
are multiple displays. Looking at the code, the host is really an Observer, and
we should probably use ObserverList for this.

+  host_ = host;
+}
+
 void MessageCenter::SetDelegate(Delegate* delegate) {
   DCHECK(!delegate_);
   delegate_ = delegate;
 }
 
 void MessageCenter::SetMessageCenterVisible(bool visible) {
   notification_list_->SetMessageCenterVisible(visible);
 }
 
 size_t MessageCenter::NotificationCount() const {
@@ skipping 110 matching lines @@
 }
 
 NotificationList* MessageCenter::GetNotificationList() {
   return notification_list_.get();
 }
 
 void MessageCenter::Delegate::OnButtonClicked(const std::string& id,
                                               int button_index) {
 }
 
+//------------------------------------------------------------------------------
+// Private.
+
+MessageCenter::MessageCenter()
+    : host_(NULL),
+      delegate_(NULL) {
+  notification_list_.reset(new NotificationList(this));
+}
+
+
 }  // namespace message_center