[gn] Allow building a snapshot with unwinding information.

src/setup-isolate-deserialize.cc

 // Copyright 2017 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 "src/setup-isolate.h"
 
 #include "src/base/logging.h"
 #include "src/interpreter/interpreter.h"
 #include "src/isolate.h"
 #include "src/ostreams.h"
 
 namespace v8 {
 namespace internal {
 
 void SetupIsolateDelegate::SetupBuiltins(Isolate* isolate,
                                          bool create_heap_objects) {
   DCHECK(!create_heap_objects);
   // No actual work to be done; builtins will be deserialized from the snapshot.
 }
 
 void SetupIsolateDelegate::SetupInterpreter(
     interpreter::Interpreter* interpreter, bool create_heap_objects) {
+#if defined(V8_USE_SNAPSHOT) && !defined(V8_USE_SNAPSHOT_WITH_UNWINDING_INFO)
+  if (FLAG_perf_prof_unwinding_info) {
+    OFStream os(stdout);
+    os << "Warning: The --perf-prof-unwinding-info flag can be passed at "
+          "mksnapshot time to get better results."
+       << std::endl;
+  }
+#endif
   DCHECK(interpreter->IsDispatchTableInitialized());
 }
 
 }  // namespace internal
 }  // namespace v8