Use -DFUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION for libFuzzer and AFL.

build/config/sanitizers/BUILD.gn

Index: build/config/sanitizers/BUILD.gn
diff --git a/build/config/sanitizers/BUILD.gn b/build/config/sanitizers/BUILD.gn
index 24055aa286840c7e115dd5490766cc23d588d2dc..845d992d6edd1c4e44c713fa1c9234f29179f79e 100644
--- a/build/config/sanitizers/BUILD.gn
+++ b/build/config/sanitizers/BUILD.gn
@@ -536,6 +536,12 @@ config("ubsan_vptr_flags") {
   }
 }
 
+config("fuzzing_build_mode") {
+  if (use_libfuzzer || use_afl) {
+    defines = [ "FUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION" ]
+  }
+}
+
 all_sanitizer_configs = [
   ":common_sanitizer_flags",
   ":coverage_flags",
@@ -550,6 +556,7 @@ all_sanitizer_configs = [
   ":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