[mojo-bindings] Support reuse of native enum classes

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..4730fc6e51f758e5fdaaa767942251464604a9e0 100644
--- a/mojo/public/tools/bindings/mojom.gni
+++ b/mojo/public/tools/bindings/mojom.gni
@@ -55,6 +55,10 @@ mojom_generator_sources = [
 #       A variant name to apply to generated bindings. Variant influences
 #       generated source filenames as wells the symbols they define.
 #
+#   cpp_only (optional)
+#       If true, only C++ bindings will be generated. This defaults to false
+#       unless |variant| is specified.
+#
 #   testonly (optional)
 #
 #   visibility (optional)
@@ -66,29 +70,38 @@ template("mojom") {
 
   cpp_sources_suffix = "cpp_sources"
   cpp_sources_target_name = "${target_name}_${cpp_sources_suffix}"
+  cpp_only = false
+  if (defined(invoker.cpp_only)) {
+    cpp_only = invoker.cpp_only
+  }
   if (defined(invoker.sources)) {
     if (defined(invoker.variant)) {
       variant = invoker.variant
+      cpp_only = true
       generator_cpp_outputs = [
         "{{source_gen_dir}}/{{source_name_part}}.mojom-${variant}.cc",
         "{{source_gen_dir}}/{{source_name_part}}.mojom-${variant}.h",
         "{{source_gen_dir}}/{{source_name_part}}.mojom-${variant}-internal.h",
       ]
-      generator_js_outputs = []
-      generator_java_outputs = []
     } else {
       generator_cpp_outputs = [
         "{{source_gen_dir}}/{{source_name_part}}.mojom.cc",
         "{{source_gen_dir}}/{{source_name_part}}.mojom.h",
         "{{source_gen_dir}}/{{source_name_part}}.mojom-internal.h",
       ]
-      generator_js_outputs =
-          [ "{{source_gen_dir}}/{{source_name_part}}.mojom.js" ]
-      generator_java_outputs =
-          [ "{{source_gen_dir}}/{{source_name_part}}.mojom.srcjar" ]
     }
   }
 
+  if (cpp_only) {
+    generator_js_outputs = []
+    generator_java_outputs = []
+  } else {
+    generator_js_outputs =
+        [ "{{source_gen_dir}}/{{source_name_part}}.mojom.js" ]
+    generator_java_outputs =
+        [ "{{source_gen_dir}}/{{source_name_part}}.mojom.srcjar" ]
+  }
+
   if (defined(invoker.sources)) {
     generator_target_name = target_name + "__generator"
     action_foreach(generator_target_name) {
@@ -123,10 +136,8 @@ template("mojom") {
         }
       }
 
-      if (defined(invoker.variant)) {
+      if (cpp_only) {
         args += [
-          "--variant",
-          invoker.variant,
           "-g",
           "c++",
         ]
@@ -137,6 +148,13 @@ template("mojom") {
         ]
       }
 
+      if (defined(invoker.variant)) {
+        args += [
+          "--variant",
+          invoker.variant,
+        ]
+      }
+
       if (defined(invoker.typemaps)) {
         foreach(typemap, invoker.typemaps) {
           args += [