[Reland] Mojo: Eliminate duplicate C API symbols

mojo/public/c/system/thunks.cc

 // Copyright 2013 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.
 
-#include "mojo/public/platform/native/system_thunks.h"
+#include "mojo/public/c/system/thunks.h"
 
 #include <assert.h>
 #include <stddef.h>
 #include <stdint.h>
 
-#include "mojo/public/platform/native/thunk_export.h"
-
 extern "C" {
 
 static MojoSystemThunks g_thunks = {0};
 
 MojoTimeTicks MojoGetTimeTicksNow() {
   assert(g_thunks.GetTimeTicksNow);
   return g_thunks.GetTimeTicksNow();
 }
 
 MojoResult MojoClose(MojoHandle handle) {
@@ skipping 247 matching lines @@
 MojoResult MojoUnwrapPlatformSharedBufferHandle(
     MojoHandle mojo_handle,
     struct MojoPlatformHandle* platform_handle,
     size_t* num_bytes,
     MojoPlatformSharedBufferHandleFlags* flags) {
   assert(g_thunks.UnwrapPlatformSharedBufferHandle);
   return g_thunks.UnwrapPlatformSharedBufferHandle(mojo_handle, platform_handle,
                                                    num_bytes, flags);
 }
 
-extern "C" THUNK_EXPORT size_t MojoSetSystemThunks(
-    const MojoSystemThunks* system_thunks) {
+}  // extern "C"
+
+size_t MojoEmbedderSetSystemThunks(const MojoSystemThunks* system_thunks) {
   if (system_thunks->size >= sizeof(g_thunks))
     g_thunks = *system_thunks;
   return sizeof(g_thunks);
 }
-
-}  // extern "C"