ipc: Add a lock to guard modifications of state of the AttachmentBroker.

ipc/attachment_broker_privileged_mac.h

Index: ipc/attachment_broker_privileged_mac.h
diff --git a/ipc/attachment_broker_privileged_mac.h b/ipc/attachment_broker_privileged_mac.h
index 329a376a9c769c24882c34881e0159bc3c70b354..a0753f2a7de8abf5aa9dd3b9e85950270745336d 100644
--- a/ipc/attachment_broker_privileged_mac.h
+++ b/ipc/attachment_broker_privileged_mac.h
@@ -123,7 +123,7 @@ class IPC_EXPORT AttachmentBrokerPrivilegedMac
   void RouteWireFormatToAnother(const MachPortWireFormat& wire_format);
 
   // The port provider must live at least as long as the AttachmentBroker.
-  base::PortProvider* port_provider_;
+  const base::PortProvider* port_provider_;
 
   DISALLOW_COPY_AND_ASSIGN(AttachmentBrokerPrivilegedMac);
 };