Clean up public interface of AttachmentBrokerUnprivileged.

ipc/attachment_broker_unprivileged.h

Index: ipc/attachment_broker_unprivileged.h
diff --git a/ipc/attachment_broker_unprivileged.h b/ipc/attachment_broker_unprivileged.h
index b572ff8266a2fbef12bb5a2aa8dcd30ee3bef397..f6d520de6e3bb1a0ad4faa455ce450bf8c93c192 100644
--- a/ipc/attachment_broker_unprivileged.h
+++ b/ipc/attachment_broker_unprivileged.h
@@ -22,17 +22,15 @@ class IPC_EXPORT AttachmentBrokerUnprivileged : public IPC::AttachmentBroker {
   AttachmentBrokerUnprivileged();
   ~AttachmentBrokerUnprivileged() 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<AttachmentBrokerUnprivileged> CreateBroker();
+  // If there is no global attachment broker, makes a new
+  // AttachmentBrokerUnprivileged and sets it as the global attachment broker.
+  // This method is thread safe.
+  static void CreateBrokerIfNeeded();
 
-  // In each unprivileged process, exactly one channel should be used to
-  // communicate brokerable attachments with the broker process.
-  void DesignateBrokerCommunicationChannel(Endpoint* endpoint);
+  // AttachmentBroker:
+  void RegisterBrokerCommunicationChannel(Endpoint* endpoint) override;
+  void DeregisterBrokerCommunicationChannel(Endpoint* endpoint) override;
+  bool IsPrivilegedBroker() override;
 
  protected:
   IPC::Sender* get_sender() { return sender_; }