Generate the proto JSON converter for DOM distiller

third_party/protobuf/proto_library.gni

Index: third_party/protobuf/proto_library.gni
diff --git a/third_party/protobuf/proto_library.gni b/third_party/protobuf/proto_library.gni
index df9d657bf46aa0366f9681f45db30cc5db87fb42..ccc2919697f3b38a8736623e2f402f0ce5c9d1f9 100644
--- a/third_party/protobuf/proto_library.gni
+++ b/third_party/protobuf/proto_library.gni
@@ -148,6 +148,15 @@ template("proto_library") {
     if (defined(invoker.deps)) {
       deps += invoker.deps
     }
+
+    if (defined(invoker.json_converter)) {
+      args += [
+        "--plugin=protoc-gen-json_converter=" +
+            rebase_path(invoker.json_converter, root_build_dir),
+        "--json_converter_out=output_dir=:$rel_out_dir",

[dcheng, 2016/06/14 04:27:39]

Seems a bit weird to specify this here, maybe we can just have an
additional_args parameter?

Also, is it possible to pas more than one --plugin argument to protoc? It'd be
nice not to have to duplicate all the protoc_plugins.

[wychen, 2016/08/07 09:16:27]

Moving all the logic to dom_distiller_js/BUILD.gn can also work, but I'm trying
to make this work as a template for protoc plugins. I'm not sure how general
this is, since we are the first one using plugins here. The benefit of putting
the logic here is that it will be visible when there are more users of the
plugins in the future. If they happen to be using json_converter, then this can
work as is. For other kinds of plugins, we'll see how much of the logic can be
factored out when the time comes.

+      ]
+      outputs += [ "$out_dir/{{source_name_part}}_json_converter.h" ]
+    }
   }
 
   source_set(target_name) {