Reland #1: Clean up interface of attachment broker.

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 3d98d495932d59ed98d6950d658c00b6a7944362..a5437361fe3f3d4737631680415ae2d0c9cd2208 100644
--- a/ipc/attachment_broker_unprivileged.h
+++ b/ipc/attachment_broker_unprivileged.h
@@ -10,6 +10,7 @@
 
 namespace IPC {
 
+class Channel;
 class Sender;
 
 // This abstract subclass of AttachmentBroker is intended for use in
@@ -19,7 +20,9 @@ class IPC_EXPORT AttachmentBrokerUnprivileged : public IPC::AttachmentBroker {
   AttachmentBrokerUnprivileged();
   ~AttachmentBrokerUnprivileged() override;
 
-  void set_sender(IPC::Sender* sender) { sender_ = sender; }
+  // In each unprivileged process, exactly one channel should be used to
+  // communicate brokerable attachments with the broker process.
+  void DesignateBrokerCommunicationChannel(IPC::Channel* channel);
 
  protected:
   IPC::Sender* get_sender() { return sender_; }