mojom: Allow mojoms to only run a subset of the bindings generators.

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 5f65c27eaf1945bfe4e0289da107ba043a44b720..41034926077026d272dee89379654e1cc30bda56 100644
--- a/mojo/public/tools/bindings/mojom.gni
+++ b/mojo/public/tools/bindings/mojom.gni
@@ -20,6 +20,9 @@
 #   testonly (optional)
 #
 #   visibility (optional)
+#
+#   generators (optional)
+#       List of languages to generate object files for.
 template("mojom") {
   assert(defined(invoker.sources),
          "\"sources\" must be defined for the $target_name template.")
@@ -103,6 +106,13 @@ template("mojom") {
     target_visibility = [ ":$target_name" ]
   }
 
+  # Work around gn requiring all outputs to be used; there are some branches
+  # through the if branches below where these variables may otherwise be unused.
+  assert(generator_cpp_outputs != [])
+  assert(generator_java_outputs != [])
+  assert(generator_js_outputs != [])
+  assert(generator_python_outputs != [])
+
   generator_target_name = target_name + "__generator"
   action_foreach(generator_target_name) {
     if (defined(invoker.visibility)) {
@@ -111,12 +121,45 @@ template("mojom") {
     script = generator_script
     inputs = generator_sources
     sources = invoker.sources
-    outputs = generator_cpp_outputs +
-              generator_java_outputs +
-              generator_js_outputs +
-              generator_python_outputs
+    outputs = []
+    generator_string = ""
+    if (defined(invoker.generators)) {
+      generator_string = "--generators="
+      first_item = true
+      foreach (item, invoker.generators) {
+        if (first_item) {
+          first_item = false
+        } else {
+          generator_string += ","
+        }
+
+        generator_string += item
+        if (item == "c++") {
+          outputs += generator_cpp_outputs
+        } else if (item == "java") {
+          outputs += generator_java_outputs
+        } else if (item == "javascript") {
+          outputs += generator_js_outputs
+        } else if (item == "python") {
+          outputs += generator_python_outputs
+        }
+      }
+    } else {
+      outputs = generator_cpp_outputs +
+                generator_java_outputs +
+                generator_js_outputs +
+                generator_python_outputs
+    }
+
     args = [
       "{{source}}",
+    ]
+
+    if (generator_string != "") {
+      args += [ generator_string ]
+    }
+
+    args += [
       "--use_chromium_bundled_pylibs",
       "-d", rebase_path("//", root_build_dir),
       "-I", rebase_path("//", root_build_dir),