[gn] Enable stricter build flags

src/d8.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include <errno.h>
 #include <stdlib.h>
 #include <string.h>
 #include <sys/stat.h>
 
 #include <algorithm>
@@ skipping 1967 matching lines @@
             .ToLocalChecked();
     Local<String> callback_name =
         v8::String::NewFromUtf8(isolate_, "receive", v8::NewStringType::kNormal)
             .ToLocalChecked();
     Local<Context> context = context_.Get(isolate_);
     Local<Value> callback =
         context->Global()->Get(context, callback_name).ToLocalChecked();
     if (callback->IsFunction()) {
       v8::TryCatch try_catch(isolate_);
       Local<Value> args[] = {message};
-      MaybeLocal<Value> result = Local<Function>::Cast(callback)->Call(
-          context, Undefined(isolate_), 1, args);
+      USE(Local<Function>::Cast(callback)->Call(context, Undefined(isolate_), 1,
+                                                args));
 #ifdef DEBUG
       if (try_catch.HasCaught()) {
         Local<Object> exception = Local<Object>::Cast(try_catch.Exception());
         Local<String> key = v8::String::NewFromUtf8(isolate_, "message",
                                                     v8::NewStringType::kNormal)
                                 .ToLocalChecked();
         Local<String> expected =
             v8::String::NewFromUtf8(isolate_,
                                     "Maximum call stack size exceeded",
                                     v8::NewStringType::kNormal)
@@ skipping 1054 matching lines @@
 }
 
 }  // namespace v8
 
 
 #ifndef GOOGLE3
 int main(int argc, char* argv[]) {
   return v8::Shell::Main(argc, argv);
 }
 #endif