Add chromium_code to tracing.gyp to build with warnings.

components/tracing.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.
 
 # This file is intentionally a gyp file rather than a gypi for dependencies
 # reasons. The other gypi files include content.gyp and content_common depends
 # on this, thus if you try to rename this to gypi and include it in
 # components.gyp, you will get a circular dependency error.
 {
+  'variables': {
+    'chromium_code': 1,
+  },
   'targets' : [
     {
       'target_name': 'tracing',
       'type': 'static_library',
-      'defines!': ['CONTENT_IMPLEMENTATION'],
       'dependencies': [
         '../base/base.gyp:base',
         '../ipc/ipc.gyp:ipc',
       ],
       'include_dirs': [
         '..',
       ],
       'sources': [
         'tracing/child_trace_message_filter.cc',
         'tracing/child_trace_message_filter.h',
         'tracing/tracing_messages.cc',
         'tracing/tracing_messages.h',
       ],
     },
   ],
 }