De-duplicate BASE_IMPLEMENTATION define in the GN build.

base/third_party/nspr/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("nspr") {
   visibility = [ "//base/*" ]
 
   sources = [
     "prtime.cc",
     "prtime.h",
   ]
 
   # In GYP this project is part of base, so it uses the base implementation
   # define. TODO(brettw) rename this define.
-  defines = [ "BASE_IMPLEMENTATION" ]
+  configs += [ "//base:base_implementation" ]
 
   if (is_android && !is_debug) {
     configs -= [ "//build/config/compiler:optimize" ]
     configs += [ "//build/config/compiler:optimize_max" ]
   }
 }