[gn] Enable stricter build flags

src/d8.cc

Index: src/d8.cc
diff --git a/src/d8.cc b/src/d8.cc
index 8745d41027e61ead49916d78fc6cc0e8d99e3ae6..dd045a4a7b4ceecee26de7638049530813dc57b3 100644
--- a/src/d8.cc
+++ b/src/d8.cc
@@ -1985,8 +1985,8 @@ class InspectorFrontend final : public v8_inspector::V8Inspector::Channel {
     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());