De-duplicate BASE_IMPLEMENTATION define in the GN build.

base/json/BUILD.gn

 # Copyright (c) 2013 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.
 
 source_set("json") {
   sources = [
     "json_file_value_serializer.cc",
     "json_file_value_serializer.h",
     "json_parser.cc",
     "json_parser.h",
     "json_reader.cc",
     "json_reader.h",
     "json_string_value_serializer.cc",
     "json_string_value_serializer.h",
     "json_value_converter.cc",
     "json_value_converter.h",
     "json_writer.cc",
     "json_writer.h",
     "string_escape.cc",
     "string_escape.h",
   ]
 
   if (is_nacl) {
     sources -= [
       "json_file_value_serializer.cc",
       "json_file_value_serializer.h",
     ]
   }
 
-  defines = [ "BASE_IMPLEMENTATION" ]
+  configs += [ "//base:base_implementation" ]
 
   deps = [
     "//base/memory",
   ]
 
   visibility = [ "//base/*" ]
 }