More SynchronousInterfacePtr conversion + some related pexe content handler cleanup.

services/nacl/nonsfi/pnacl_link.mojom

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 module mojo.nacl;
 
 [ServiceName="mojo::nacl::PexeLinkerInit"]
 interface PexeLinkerInit {
-  PexeLinkerStart(handle<message_pipe> message_handle);
+  PexeLinkerStart(PexeLinker& linker_request);
 };
 
 interface PexeLinker {
   PexeLink(array<string> object_file_names) => (string nexe_file_name);
 };