Reland #1: Clean up interface of attachment broker.

ipc/ipc_channel_reader.cc

Index: ipc/ipc_channel_reader.cc
diff --git a/ipc/ipc_channel_reader.cc b/ipc/ipc_channel_reader.cc
index 7e8902e54d2a894a9cfe65df489e3f999c9386c5..30f03a1f2decbb2fd74b690baf5cc22eeb392ef5 100644
--- a/ipc/ipc_channel_reader.cc
+++ b/ipc/ipc_channel_reader.cc
@@ -163,9 +163,18 @@ void ChannelReader::DispatchMessage(Message* m) {
                IPC_MESSAGE_ID_LINE(m->type()));
 #endif
   m->TraceMessageEnd();
-  if (IsInternalMessage(*m))
+
+  bool handled = false;
+  if (IsInternalMessage(*m)) {
     HandleInternalMessage(*m);
-  else
+    handled = true;
+  }
+#if USE_ATTACHMENT_BROKER
+  if (!handled && IsAttachmentBrokerEndpoint() && GetAttachmentBroker()) {
+    handled = GetAttachmentBroker()->OnMessageReceived(*m);
+  }
+#endif  // USE_ATTACHMENT_BROKER
+  if (!handled)
     listener_->OnMessageReceived(*m);
   if (m->dispatch_error())
     listener_->OnBadMessageReceived(*m);