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

mojo/public/tools/bindings/mojom.gni

Index: mojo/public/tools/bindings/mojom.gni
diff --git a/mojo/public/tools/bindings/mojom.gni b/mojo/public/tools/bindings/mojom.gni
index 9cd72173a0aa9d7fc8728028eb34f96aa8e42b72..650dbd81ef978306a26a1cb090d248ec3b1d9022 100644
--- a/mojo/public/tools/bindings/mojom.gni
+++ b/mojo/public/tools/bindings/mojom.gni
@@ -58,6 +58,11 @@ mojom_generator_sources = [
 #   testonly (optional)
 #
 #   visibility (optional)
+#
+#   TODO(yzshen): add comments.
+#   cpp_dll_export_declaration (optional)
+#   cpp_dll_export_include (optional)
+#   cpp_configs (optional)
 template("mojom") {
   assert(
       defined(invoker.sources) || defined(invoker.deps) ||
@@ -145,6 +150,20 @@ template("mojom") {
           ]
         }
       }
+
+      if (defined(invoker.dll_export_declaration)) {
+        args += [
+          "--dll_export_declaration",
+          invoker.dll_export_declaration,
+        ]
+      }
+
+      if (defined(invoker.dll_export_include)) {
+        args += [
+          "--dll_export_include",
+          invoker.dll_export_include,
+        ]
+      }
     }
   }
 
@@ -208,6 +227,10 @@ template("mojom") {
     # this target *except* mojo/public/cpp/bindings and other *_cpp_sources
     # targets.
     source_set(cpp_sources_target_name) {
+      if (defined(invoker.cpp_configs)) {
+        configs += invoker.cpp_configs
+      }
+
       if (defined(invoker.testonly)) {
         testonly = invoker.testonly
       }