ipc: Move AttachmentBrokerPrivileged singleton logic into ipc/.

ipc/attachment_broker_privileged.h

Index: ipc/attachment_broker_privileged.h
diff --git a/ipc/attachment_broker_privileged.h b/ipc/attachment_broker_privileged.h
index 387dbe5c9f77b714ccf08f511fd74c7a1c1c2c95..b2ee01aeb2dc7e3ba9a25ff002f3abb632101a47 100644
--- a/ipc/attachment_broker_privileged.h
+++ b/ipc/attachment_broker_privileged.h
@@ -25,13 +25,10 @@ class IPC_EXPORT AttachmentBrokerPrivileged : public IPC::AttachmentBroker {
   AttachmentBrokerPrivileged();
   ~AttachmentBrokerPrivileged() override;
 
-   // On platforms that support attachment brokering, returns a new instance of
-   // a platform-specific attachment broker. Otherwise returns |nullptr|.
-   // The caller takes ownership of the newly created instance, and is
-   // responsible for ensuring that the attachment broker lives longer than
-   // every IPC::Channel. The new instance automatically registers itself as the
-   // global attachment broker.
-  static scoped_ptr<AttachmentBrokerPrivileged> CreateBroker();
+  // If there is no global attachment broker, makes a new
+  // AttachmentBrokerPrivileged and sets it as the global attachment broker.
+  // This method is thread safe.
+  static void CreateBrokerIfNeeded();
 
   // AttachmentBroker overrides.
   void RegisterCommunicationChannel(Endpoint* endpoint) override;