Make the root_namespace argument to json_schema_compiler.gypi a string

build/json_schema_compile.gypi

 # Copyright (c) 2012 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.
 
 {
   'variables': {
     # When including this gypi, the following variables must be set:
-    #   schema_files: a list of json or IDL files that comprise the api model.
-    #   cc_dir: path to generated files
-    #   root_namespace: the C++ namespace that all generated files go under
+    #   schema_files:
+    #     An array of json or idl files that comprise the api model.
+    #   cc_dir:
+    #     The directory to put the generated code in.
+    #   root_namespace:
+    #     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".
+    #
     # Functions and namespaces can be excluded by setting "nocompile" to true.
     # The default root path of API implementation sources is
     # chrome/browser/extensions/api and can be overridden by setting "impl_dir".
     'api_gen_dir': '<(DEPTH)/tools/json_schema_compiler',
     'api_gen': '<(api_gen_dir)/compiler.py',
     'impl_dir%': 'chrome/browser/extensions/api',
   },
   'rules': [
     {
       # GN version: //extensions/generated_extensions_api.gni
@@ skipping 84 matching lines @@
   ],
   'direct_dependent_settings': {
     'include_dirs': [
       '<(SHARED_INTERMEDIATE_DIR)',
     ]
   },
   # This target exports a hard dependency because it generates header
   # files.
   'hard_dependency': 1,
 }