Add support for Attachment Brokering of IPC::Channels on multiple threads.

ipc/attachment_broker.cc

Index: ipc/attachment_broker.cc
diff --git a/ipc/attachment_broker.cc b/ipc/attachment_broker.cc
index 45fe1204681395aefc7218e8475dcdfbd784b2e4..3e73ab3f7f10e472c0583a7d3bc472160545bb00 100644
--- a/ipc/attachment_broker.cc
+++ b/ipc/attachment_broker.cc
@@ -71,7 +71,9 @@ void AttachmentBroker::RemoveObserver(AttachmentBroker::Observer* observer) {
     observers_.erase(it);
 }
 
-void AttachmentBroker::RegisterCommunicationChannel(Endpoint* endpoint) {
+void AttachmentBroker::RegisterCommunicationChannel(
+    Endpoint* endpoint,
+    scoped_refptr<base::SingleThreadTaskRunner> runner) {
   NOTREACHED();
 }