Use -DFUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION for libFuzzer and AFL.

build/config/sanitizers/BUILD.gn

 # Copyright 2014 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.
 
 import("//build_overrides/build.gni")
 import("//build/config/chrome_build.gni")
 import("//build/config/chromecast_build.gni")
 import("//build/config/clang/clang.gni")
 import("//build/config/sanitizers/sanitizers.gni")
 import("//build/toolchain/toolchain.gni")
@@ skipping 518 matching lines @@
       ubsan_vptr_blacklist_path =
           rebase_path("//tools/ubsan/vptr_blacklist.txt", root_build_dir)
     }
     cflags = [
       "-fsanitize=vptr",
       "-fsanitize-blacklist=$ubsan_vptr_blacklist_path",
     ]
   }
 }
 
+config("fuzzing_build_mode") {
+  if (use_libfuzzer || use_afl) {
+    defines = [ "FUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION" ]
+  }
+}
+
 all_sanitizer_configs = [
   ":common_sanitizer_flags",
   ":coverage_flags",
   ":default_sanitizer_ldflags",
   ":asan_flags",
   ":cfi_flags",
   ":lsan_flags",
   ":msan_flags",
   ":tsan_flags",
   ":ubsan_flags",
   ":ubsan_no_recover",
   ":ubsan_null_flags",
   ":ubsan_security_flags",
   ":ubsan_vptr_flags",
+  ":fuzzing_build_mode",
 ]
 
 # This config is applied by default to all targets. It sets the compiler flags
 # for sanitizer usage, or, if no sanitizer is set, does nothing.
 #
 # This needs to be in a separate config so that targets can opt out of
 # sanitizers (by removing the config) if they desire. Even if a target
 # removes this config, executables & shared libraries should still depend on
 # :deps if any of their dependencies have not opted out of sanitizers.
 # Keep this list in sync with default_sanitizer_flags_but_ubsan_vptr.
 config("default_sanitizer_flags") {
   configs = all_sanitizer_configs
 }
 
 # This config is equivalent to default_sanitizer_flags, but excludes ubsan_vptr.
 # This allows to selectively disable ubsan_vptr, when needed. In particular,
 # if some third_party code is required to be compiled without rtti, which
 # is a requirement for ubsan_vptr.
 config("default_sanitizer_flags_but_ubsan_vptr") {
   configs = all_sanitizer_configs - [ ":ubsan_vptr_flags" ]
 }
 
 config("default_sanitizer_flags_but_coverage") {
   configs = all_sanitizer_configs - [ ":coverage_flags" ]
 }