ipc: Use a global for the process's attachment broker.

ipc/attachment_broker.h

Index: ipc/attachment_broker.h
diff --git a/ipc/attachment_broker.h b/ipc/attachment_broker.h
index 5b65289393706545e221d7f05b2df9f615b88068..b77700836423782fd2100e269f23f953a6b45697 100644
--- a/ipc/attachment_broker.h
+++ b/ipc/attachment_broker.h
@@ -50,6 +50,11 @@ class IPC_EXPORT AttachmentBroker : public Listener {
         const BrokerableAttachment::AttachmentId& id) = 0;
   };
 
+  // Each process has at most one attachment broker. The process is responsible
+  // for ensuring that |broker| stays alive for the life time of the process.

[Tom Sepez, 2015/09/11 15:58:30]

nit: lifetime

[erikchen, 2015/09/11 20:16:02]

I updated the comment to say:
"""
Each process has at most one attachment broker. The process is responsible
for ensuring that |broker| stays alive for as long as the process is
sending/receiving ipc messages.
"""

+  static void SetGlobal(AttachmentBroker* broker);
+  static AttachmentBroker* GetGlobal();
+
   AttachmentBroker();
   ~AttachmentBroker() override;