Reland #1: Clean up interface of attachment broker.

ipc/ipc_channel_reader.h

Index: ipc/ipc_channel_reader.h
diff --git a/ipc/ipc_channel_reader.h b/ipc/ipc_channel_reader.h
index 77ef382ce54b5a329a097bde56cd89123c230f1b..6f4b870df0da00f80fe57ab79e639e80a0b85bd3 100644
--- a/ipc/ipc_channel_reader.h
+++ b/ipc/ipc_channel_reader.h
@@ -117,6 +117,9 @@ class IPC_EXPORT ChannelReader : public SupportsAttachmentBrokering,
   // Get the process ID for the sender of the message.
   virtual base::ProcessId GetSenderPID() = 0;
 
+  // Whether the channel is an endpoint of attachment brokering.
+  virtual bool IsAttachmentBrokerEndpoint() = 0;
+
  private:
   FRIEND_TEST_ALL_PREFIXES(ChannelReaderTest, AttachmentAlreadyBrokered);
   FRIEND_TEST_ALL_PREFIXES(ChannelReaderTest, AttachmentNotYetBrokered);