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

mojo/public/tools/bindings/mojom.gni

 # 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.
 
 mojom_generator_root = "//mojo/public/tools/bindings"
 mojom_generator_script = "$mojom_generator_root/mojom_bindings_generator.py"
 mojom_generator_sources = [
   "$mojom_generator_root/generators/mojom_cpp_generator.py",
   "$mojom_generator_root/generators/mojom_js_generator.py",
   "$mojom_generator_root/generators/mojom_java_generator.py",
@@ skipping 40 matching lines @@
 #   typemaps (optional)
 #       A list of typemap files to apply during bindings generation.
 #
 #   variant (optional)
 #       A variant name to apply to generated bindings. Variant influences
 #       generated source filenames as wells the symbols they define.
 #
 #   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) ||
           defined(invoker.public_deps),
       "\"sources\" or \"deps\" must be defined for the $target_name template.")
 
   cpp_sources_suffix = "cpp_sources"
   cpp_sources_target_name = "${target_name}_${cpp_sources_suffix}"
   if (defined(invoker.sources)) {
     if (defined(invoker.variant)) {
@@ skipping 67 matching lines @@
       }
 
       if (defined(invoker.typemaps)) {
         foreach(typemap, invoker.typemaps) {
           args += [
             "--typemap",
             rebase_path(typemap, root_build_dir),
           ]
         }
       }
+
+      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,
+        ]
+      }
     }
   }
 
   source_set(target_name) {
     if (defined(invoker.visibility)) {
       visibility = invoker.visibility
     }
     if (defined(invoker.testonly)) {
       testonly = invoker.testonly
     }
@@ skipping 43 matching lines @@
   }
 
   if (defined(invoker.sources)) {
     # The generated C++ source files. The main reason to introduce this target
     # is so that mojo/public/cpp/bindings can depend on mojom interfaces without
     # circular dependencies. It means that the target is missing the dependency
     # on mojo/public/cpp/bindings. No external targets should depend directly on
     # 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
       }
       sources = process_file_template(invoker.sources, generator_cpp_outputs)
       deps = [
         ":$generator_target_name",
         "//base",
         "//mojo/public/interfaces/bindings:bindings__generator",
       ]
       foreach(d, all_deps) {
@@ skipping 44 matching lines @@
         # //mojo/something:something and we can append "_java" to get the java
         # dependency name.
         full_name = get_label_info(d, "label_no_toolchain")
         deps += [ "${full_name}_java" ]
       }
 
       srcjar_deps = [ ":$java_srcjar_target_name" ]
     }
   }
 }