More SynchronousInterfacePtr conversion + some related pexe content handler cleanup.

services/nacl/nonsfi/pnacl_compile.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::PexeCompilerInit"]
 interface PexeCompilerInit {
-  PexeCompilerStart(handle<message_pipe> message_handle);
+  PexeCompilerStart(PexeCompiler& compiler_request);
 };
 
 interface PexeCompiler {
   PexeCompile(string pexe_file_name) => (array<string> object_file_names);
 };