IPC: Add new classes required for attachment brokering.

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 d4e52606776ff043cd5afc81bd52f9df7e9c06a7..62fcb191a43bfc23c07b12483cc0a7f6be41a437 100644
--- a/ipc/mojo/ipc_channel_mojo.cc
+++ b/ipc/mojo/ipc_channel_mojo.cc
@@ -532,6 +532,9 @@ MojoResult ChannelMojo::ReadFromMessageAttachmentSet(
                   attachment.get())->TakeHandle();
           handles->push_back(handle.release().value());
         } break;
+        case MessageAttachment::TYPE_WIN_HANDLE:
+          NOTREACHED();
+          break;
       }
     }