De-duplicate BASE_IMPLEMENTATION define in the GN build.

base/debug/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("debug") {
   sources = [
     "alias.cc",
     "alias.h",
     "asan_invalid_access.cc",
     "asan_invalid_access.h",
@@ skipping 37 matching lines @@
 
   if (is_nacl) {
     sources -= [
       "crash_logging.cc",
       "crash_logging.h",
       "stack_trace.cc",
       "stack_trace_posix.cc",
     ]
   }
 
-  defines = [ "BASE_IMPLEMENTATION" ]
+  configs += [ "//base:base_implementation" ]
 
   deps = [
     "//base/memory",
     "//base/process",
   ]
 
   if (is_linux) {
-    defines += [ "USE_SYMBOLIZE" ]
+    defines = [ "USE_SYMBOLIZE" ]
     deps += [ "//base/third_party/symbolize" ]
   }
 
   allow_circular_includes_from = [
     "//base/memory",
     "//base/process",
   ]
 
   visibility = [ "//base/*" ]
 }