Reland #1: Clean up interface of attachment broker.

ipc/ipc_channel_posix.cc

Index: ipc/ipc_channel_posix.cc
diff --git a/ipc/ipc_channel_posix.cc b/ipc/ipc_channel_posix.cc
index 1dd5702a229db75110c4ce97e0959f24c866ec29..4a3eb9d59dbaf853394e04393d93883e8e745190 100644
--- a/ipc/ipc_channel_posix.cc
+++ b/ipc/ipc_channel_posix.cc
@@ -955,6 +955,10 @@ base::ProcessId ChannelPosix::GetSenderPID() {
   return GetPeerPID();
 }
 
+bool ChannelPosix::IsAttachmentBrokerEndpoint() {
+  return is_attachment_broker_endpoint();
+}
+
 void ChannelPosix::Close() {
   // Close can be called multiple time, so we need to make sure we're
   // idempotent.