[Draft] try out the idea of adding export macros for C++ bindings.

mojo/public/cpp/bindings/tests/struct_unittest.cc

 // 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.
 
 #include <stddef.h>
 #include <stdint.h>
 #include <string.h>
 #include <utility>
 
 #include "mojo/public/cpp/bindings/lib/fixed_buffer.h"
 #include "mojo/public/cpp/system/message_pipe.h"
 #include "mojo/public/interfaces/bindings/tests/test_structs.mojom.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
 namespace mojo {
 namespace test {
 namespace {
 
-RectPtr MakeRect(int32_t factor = 1) {
+RectPtr __declspec(dllexport) MakeRect(int32_t factor = 1) {
   RectPtr rect(Rect::New());
   rect->x = 1 * factor;
   rect->y = 2 * factor;
   rect->width = 10 * factor;
   rect->height = 20 * factor;
   return rect;
 }
 
 void CheckRect(const Rect& rect, int32_t factor = 1) {
   EXPECT_EQ(1 * factor, rect.x);
@@ skipping 379 matching lines @@
     expected_output->f_int32 = 123;
 
     MultiVersionStructV0Ptr output =
         SerializeAndDeserialize<MultiVersionStructV0Ptr>(std::move(input));
     EXPECT_TRUE(output);
     EXPECT_TRUE(output->Equals(*expected_output));
   }
 }
 }  // namespace test
 }  // namespace mojo