Reland #1: Clean up interface of attachment broker.

ipc/ipc_channel.h

Index: ipc/ipc_channel.h
diff --git a/ipc/ipc_channel.h b/ipc/ipc_channel.h
index c5c46a78048c1df8ce3d2e23bdf5a76a9ac83e24..a9f70b24e34c9136401e755a973b26d236ab433f 100644
--- a/ipc/ipc_channel.h
+++ b/ipc/ipc_channel.h
@@ -165,6 +165,7 @@ class IPC_EXPORT Channel : public Sender {
       Listener* listener,
       AttachmentBroker* broker = nullptr);
 
+  Channel() : attachment_broker_endpoint_(false) {}
   ~Channel() override;
 
   // Connect the pipe.  On the server side, this will initiate
@@ -252,6 +253,17 @@ class IPC_EXPORT Channel : public Sender {
   static void NotifyProcessForkedForTesting();
 #endif
 
+  void set_attachment_broker_endpoint(bool is_endpoint) {
+    attachment_broker_endpoint_ = is_endpoint;
+  }
+
+ protected:
+  bool is_attachment_broker_endpoint() { return attachment_broker_endpoint_; }
+
+ private:
+  // Whether this channel is used as an endpoint for sending and receiving
+  // brokerable attachment messages to/from the broker process.
+  bool attachment_broker_endpoint_;
 };
 
 #if defined(OS_POSIX)