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

mojo/public/interfaces/bindings/tests/BUILD.gn

 # 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.
 
 import("../../../tools/bindings/mojom.gni")
 
 mojom("test_interfaces") {
   testonly = true
   sources = [
     "math_calculator.mojom",
     "no_module.mojom",
     "ping_service.mojom",
     "rect.mojom",
     "regression_tests.mojom",
     "sample_factory.mojom",
     "sample_import.mojom",
     "sample_import2.mojom",
     "sample_interfaces.mojom",
     "sample_service.mojom",
     "scoping.mojom",
     "serialization_test_structs.mojom",
     "test_constants.mojom",
     "test_native_types.mojom",
     "test_structs.mojom",
     "validation_test_interfaces.mojom",
   ]
 
   with_environment = false
+
+  dll_export_declaration = "CONTENT_EXPORT"
+  dll_export_include = "content/common/content_export.h"
+  cpp_configs = [ "//content:content_implementation" ]
 }
 
 mojom("test_interfaces_experimental") {
   testonly = true
   sources = [
     "test_unions.mojom",
   ]
 
   with_environment = false
 }
@@ skipping 56 matching lines @@
 
   variant = "blink"
   typemaps = [ "blink_test.typemap" ]
 
   with_environment = false
 
   public_deps = [
     ":test_interfaces",
   ]
 }