Revert of 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 068bf8e2b0bac94366acca0debca922d823fa960..b572ff8266a2fbef12bb5a2aa8dcd30ee3bef397 100644
--- a/ipc/attachment_broker_unprivileged.h
+++ b/ipc/attachment_broker_unprivileged.h
@@ -22,14 +22,17 @@
   AttachmentBrokerUnprivileged();
   ~AttachmentBrokerUnprivileged() override;
 
-  // 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();
+   // 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();
 
-  // AttachmentBroker:
-  void DesignateBrokerCommunicationChannel(Endpoint* endpoint) override;
-  bool IsPrivilegedBroker() override;
+  // In each unprivileged process, exactly one channel should be used to
+  // communicate brokerable attachments with the broker process.
+  void DesignateBrokerCommunicationChannel(Endpoint* endpoint);
 
  protected:
   IPC::Sender* get_sender() { return sender_; }