De-duplicate BASE_IMPLEMENTATION define in the GN build.

base/trace_event/BUILD.gn

 # Copyright (c) 2015 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("trace_event") {
   sources = [
     "memory_dump_manager.cc",
     "memory_dump_manager.h",
     "memory_dump_provider.h",
     "process_memory_dump.cc",
@@ skipping 23 matching lines @@
     "trace_event_win.h",
   ]
 
   if (is_nacl) {
     sources -= [
       "process_memory_totals_dump_provider.cc",
       "trace_event_system_stats_monitor.cc",
     ]
   }
 
-  defines = [ "BASE_IMPLEMENTATION" ]
+  configs += [ "//base:base_implementation" ]
 
   deps = [
     "//base/debug",
     "//base/json",
     "//base/memory",
     "//base/process",
     "//base/third_party/dynamic_annotations",
   ]
 
   allow_circular_includes_from = [
     "//base/debug",
     "//base/memory",
     "//base/process",
   ]
 
   visibility = [ "//base/*" ]
 }