Fix declaration of MojoMain and remove win32 defines

mojo/public/c/system/main.h

 // Copyright 2014 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.
 
 #ifndef MOJO_PUBLIC_C_SYSTEM_MAIN_H_
 #define MOJO_PUBLIC_C_SYSTEM_MAIN_H_
 
 #include "mojo/public/c/system/types.h"
 
 // Implement MojoMain directly as the entry point for an application.
 //
 // MojoResult MojoMain(MojoHandle application_request) {
 //   ...
 // }
-//
-// TODO(davemoore): Establish this as part of our SDK for third party mojo
-// application writers.
 
 #if defined(__cplusplus)
 extern "C" {
 #endif
 
-#if defined(WIN32)
-__declspec(dllexport) MojoResult
-    __cdecl MojoMain(MojoHandle application_request);
-#else  // !defined(WIN32)
-__attribute__((visibility("default"))) MojoResult
-    MojoMain(MojoHandle service_provider_handle);
-#endif  // defined(WIN32)
+__attribute__((visibility("default"))) MojoResult MojoMain(
+    MojoHandle application_request);
 
 #ifdef __cplusplus
 }  // extern "C"
 #endif
 
 #endif  // MOJO_PUBLIC_C_SYSTEM_MAIN_H_