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

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

Index: mojo/public/interfaces/bindings/tests/BUILD.gn
diff --git a/mojo/public/interfaces/bindings/tests/BUILD.gn b/mojo/public/interfaces/bindings/tests/BUILD.gn
index d0af97059b2921a52c39e29b79f9ea63fd482c8d..17f560cadc19fa1b124f3469871e394b17f79b56 100644
--- a/mojo/public/interfaces/bindings/tests/BUILD.gn
+++ b/mojo/public/interfaces/bindings/tests/BUILD.gn
@@ -26,6 +26,10 @@ mojom("test_interfaces") {
   ]
 
   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") {