Make ui gn check pass. Misc GN build improvements.

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.
 
 # Generate C++ and JavaScript source files from mojom files. The output files
 # will go under the generated file directory tree with the same path as each
 # input file.
 #
 # Parameters:
 #
 #   sources (required)
 #       List of source .mojom files to compile.
 #
 #   deps (optional)
+#
 #   visibility (optional)
-#       Normal meaning.
+#       Normal meaning. However, this must be a list (normal visibility
+#       allows a single string).
 template("mojom") {
   assert(defined(invoker.sources),
          "\"sources\" must be defined for the $target_name template.")
 
   generator_root = "//mojo/public/tools/bindings"
   generator_script = "$generator_root/mojom_bindings_generator.py"
   generator_sources = [
     generator_script,
     "$generator_root/generators/cpp_templates/enum_declaration.tmpl",
     "$generator_root/generators/cpp_templates/interface_declaration.tmpl",
@@ skipping 41 matching lines @@
     "{{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_python_outputs = [
     "{{source_gen_dir}}/{{source_name_part}}_mojom.py",
   ]
 
-  target_visibility = ":$target_name"
+  if (defined(invoker.visibility)) {
+    # Need to save this because the the target_name is overwritten inside the
+    # action to be that of the action itself. Only define this in the case the
+    # var is used to avoid unused var error.
+    target_visibility = ":$target_name"
+  }
 
   generator_target_name = target_name + "_generator"
   action_foreach(generator_target_name) {
-    visibility = target_visibility
+    if (defined(invoker.visibility)) {
+      visibility = [ target_visibility ] + invoker.visibility  # If you get an error here, make your visibility be a list.
+    }
     script = generator_script
     inputs = generator_sources
     sources = invoker.sources
     outputs = generator_cpp_outputs +
               generator_js_outputs +
               generator_python_outputs
     args = [
       "{{source}}",
       "--use_chromium_bundled_pylibs",
       "-d", rebase_path("//", root_build_dir),
@@ skipping 10 matching lines @@
     data = process_file_template(invoker.sources, generator_js_outputs)
     deps = [
       ":$generator_target_name",
       "//mojo/public/cpp/bindings",
     ]
     if (defined(invoker.deps)) {
       deps += invoker.deps
     }
   }
 }