Remove the hard_dep flag in GN.

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.
 template("mojom") {
   assert(defined(invoker.sources),
          "\"sources\" must be defined for the $target_name template.")
 
   generator_root = "//mojo/public/tools/bindings"
@@ skipping 44 matching lines @@
   ]
   generator_js_outputs = [
     "$target_gen_dir/{{source_name_part}}.mojom.js",
   ]
 
   target_visibility = ":$target_name"
 
   generator_target_name = target_name + "_generator"
   action_foreach(generator_target_name) {
     visibility = target_visibility
-    hard_dep = true
     script = generator_script
     source_prereqs = generator_sources
     sources = invoker.sources
     outputs = generator_cpp_outputs + generator_js_outputs
     args = [
       "{{source}}",
       "--use_chromium_bundled_pylibs",
       "-d", rebase_path("//", root_build_dir),
       "-o", rebase_path(target_gen_dir, root_build_dir),
     ]
   }
 
   source_set(target_name) {
-    hard_dep = true

[scottmg, 2014/05/13 23:21:22]

sorry, i forget, just to confirm source_set got auto-on'd, right?

     sources = process_file_template(invoker.sources, generator_cpp_outputs)
     data = process_file_template(invoker.sources, generator_js_outputs)
     deps = [
       ":$generator_target_name",
       "//mojo/public/cpp/bindings",
     ]
   }
 }