Make the root_namespace argument to json_schema_compiler.gypi a string

chrome/browser/performance_monitor/performance_monitor.gyp

 # 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.
 
 {
   'targets': [
     {
       'target_name': 'performance_monitor',
       'type': 'static_library',
       'sources': [
         '<@(schema_files)',
       ],
       'includes': [
         '../../../build/json_schema_compile.gypi',
       ],
       'variables': {
         'chromium_code': 1,
         'schema_files': [
           'events.json',
         ],
         'cc_dir': 'chrome/browser/performance_monitor',
-        'root_namespace': 'performance_monitor',
+        'root_namespace': 'performance_monitor::%(namespace)s',
       },
     },
   ],
 }