Stub in more IPC attachment brokering functionality.

ipc/mojo/ipc_channel_mojo.cc

Index: ipc/mojo/ipc_channel_mojo.cc
diff --git a/ipc/mojo/ipc_channel_mojo.cc b/ipc/mojo/ipc_channel_mojo.cc
index 4b0916d25b98ee736b19fe2fba04d2d3b3deba3d..66d280d47f7dd6d0789a60da4fb97393d5d1bee2 100644
--- a/ipc/mojo/ipc_channel_mojo.cc
+++ b/ipc/mojo/ipc_channel_mojo.cc
@@ -524,7 +524,9 @@ MojoResult ChannelMojo::ReadFromMessageAttachmentSet(
                   attachment.get())->TakeHandle();
           handles->push_back(handle.release().value());
         } break;
-        case MessageAttachment::TYPE_WIN_HANDLE:
+        case MessageAttachment::TYPE_BROKERABLE_ATTACHMENT:
+          // Brokerable attachments are handled by the AttachmentBroker so
+          // there's no need to do anything here.
           NOTREACHED();
           break;
       }