Fix an attachment broker race condition.

ipc/attachment_broker.h

Index: ipc/attachment_broker.h
diff --git a/ipc/attachment_broker.h b/ipc/attachment_broker.h
index 70383edf71368737164e2fbcd0415ed8bcb0f945..a106e297ef3764fb09b66fae5098e19542fa258e 100644
--- a/ipc/attachment_broker.h
+++ b/ipc/attachment_broker.h
@@ -107,6 +107,10 @@ class IPC_EXPORT AttachmentBroker : public Listener {
   virtual void RegisterBrokerCommunicationChannel(Endpoint* endpoint);
   virtual void DeregisterBrokerCommunicationChannel(Endpoint* endpoint);
 
+  // Informs the attachment broker that a channel endpoint has received its
+  // peer's PID.
+  virtual void ReceivedPeerPid(base::ProcessId peer_pid);
+
   // True if and only if this broker is privileged.
   virtual bool IsPrivilegedBroker();