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

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

Index: mojo/public/cpp/bindings/tests/struct_unittest.cc
diff --git a/mojo/public/cpp/bindings/tests/struct_unittest.cc b/mojo/public/cpp/bindings/tests/struct_unittest.cc
index a1041e531e6d2ba4e97e38b5e94a8b80b87c6b37..c6a05f9f63b62c71b393d01d958404238a70547e 100644
--- a/mojo/public/cpp/bindings/tests/struct_unittest.cc
+++ b/mojo/public/cpp/bindings/tests/struct_unittest.cc
@@ -16,7 +16,7 @@ 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;