Make the root_namespace argument to json_schema_compiler.gypi a string

extensions/generated_extensions_api.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.
 
 # Defines a static library corresponding to the output of schema compiler tools
 # over a set of extensions API schemas (IDL or JSON format.) The library target
 # has implicit hard dependencies on all schema files listed by the invoker and
 # is itself a hard dependency.
 #
 # Invocations of this template may use the following variables:
 #
 # sources [required] A list of schema files to be compiled.
 #
 # root_namespace [required]
-#   The namespace in which generated API code is to be
-#   wrapped. C++ namespace syntax is accepted for nested namespace
-#   (e.g. "foo::bar::api").
+#     A Python string substituion pattern used to generate the C++
+#     namespace for each API. Use %(namespace)s to replace with the API
+#     namespace, like "toplevel::%(namespace)s_api".
 #
 # bundle [optional, default = false]
 #   Boolean indicating if the schema files should be bundled or not.
 #
 # impl_dir [required if bundle = true, otherwise unused]
 #   The path containing C++ implementations of API functions. This path is
 #   used as the root path when looking for {schema}/{schema}_api.h headers
 #   during the API bundle generation phase. Such headers, if found, are
 #   automatically included by the generated code.
 #
@@ skipping 113 matching lines @@
     direct_dependent_configs = [ ":$generated_config_name" ]
 
     if (defined(invoker.visibility)) {
       visibility = invoker.visibility
     }
     if (defined(invoker.output_name)) {
       output_name = invoker.output_name
     }
   }
 }