Mojo JS bindings: add support to generate "dual-mode" code.

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 b0bcce8058d8371dc59e7b0d279d535dec454b51..081636afa9e1941c87f5a217e96281f0d4b3ed4e 100644
--- a/mojo/public/tools/bindings/mojom.gni
+++ b/mojo/public/tools/bindings/mojom.gni
@@ -158,6 +158,15 @@ if (enable_mojom_typemapping) {
 #       TODO(yzshen): Convert all users to use new JS bindings and remove the
 #       old mode.
 #
+#   js_bindings_mode (optional)
+#       |js_bindings_mode| and |use_new_js_bindings| are mutually exclusive. You
+#       can only specify at most one of them.
+#       - "new" (default): generate only the new-style JS bindings;
+#       - "both": generate both the old- and new-style JS bindings;
+#       - "old": generate only the old-style JS bindings.
+#       TODO(yzshen): Convert all users to use new JS bindings and remove the
+#       old mode.
+#
 #   component_output_prefix (optional)
 #       The prefix to use for the output_name of any component library emitted
 #       for generated C++ bindings. If this is omitted, C++ bindings targets are
@@ -212,6 +221,12 @@ template("mojom") {
       defined(invoker.sources) || defined(invoker.deps) ||
           defined(invoker.public_deps),
       "\"sources\" or \"deps\" must be defined for the $target_name template.")
+
+  assert(!defined(invoker.use_new_js_bindings) ||
+             !defined(invoker.js_bindings_mode),
+         "You shouldn't specify both \"use_new_js_bindings\" and " +
+             "\"js_bindings_mode\".")
+
   if (defined(invoker.export_class_attribute) ||
       defined(invoker.export_define) || defined(invoker.export_header)) {
     assert(defined(invoker.export_class_attribute))
@@ -238,10 +253,16 @@ template("mojom") {
     assert(defined(invoker.component_deps_blink))
   }
 
-  if (defined(invoker.use_new_js_bindings)) {
-    assert(invoker.use_new_js_bindings || !invoker.use_new_js_bindings,
-           "Surpress unused variable error when JS bindings are not needed.")
+  js_bindings_mode = "new"
+  if (defined(invoker.js_bindings_mode)) {
+    js_bindings_mode = invoker.js_bindings_mode
   }
+  if (defined(invoker.use_new_js_bindings) && !invoker.use_new_js_bindings) {
+    js_bindings_mode = "old"
+  }
+  assert(js_bindings_mode == "new" || js_bindings_mode == "both" ||
+             js_bindings_mode == "old",
+         "Invalid js_bindings_mode value.")
 
   all_deps = []
   if (defined(invoker.deps)) {
@@ -877,10 +898,10 @@ template("mojom") {
           "javascript",
         ]
 
-        if (!defined(invoker.use_new_js_bindings) ||
-            invoker.use_new_js_bindings) {
-          args += [ "--use_new_js_bindings" ]
-        }
+        args += [
+          "--js_bindings_mode",
+          js_bindings_mode,
+        ]
       }
     }