[d8] Fix locking bug w/ externalized contents

|Shell::externalized_contents_| is guarded by |Shell::workers_mutex_|,
but wasn't being acquired when the serialize WriteValue call failed.

Bug: v8:8034
Change-Id: Idd0448e9f44d6b26c17987405d5d7394449e8bb3
Reviewed-on: https://chromium-review.googlesource.com/1170316Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Ben Smith <binji@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55060}

src/d8.cc

@@ -3278,15 +3278,14 @@ std::unique_ptr<SerializationData> Shell::SerializeValue(
   bool ok;
   Local<Context> context = isolate->GetCurrentContext();
   Serializer serializer(isolate);
+  std::unique_ptr<SerializationData> data;
   if (serializer.WriteValue(context, value, transfer).To(&ok)) {
-    std::unique_ptr<SerializationData> data = serializer.Release();
-    base::LockGuard<base::Mutex> lock_guard(workers_mutex_.Pointer());
-    serializer.AppendExternalizedContentsTo(&externalized_contents_);
-    return data;
+    data = serializer.Release();
   }
   // Append externalized contents even when WriteValue fails.
+  base::LockGuard<base::Mutex> lock_guard(workers_mutex_.Pointer());
   serializer.AppendExternalizedContentsTo(&externalized_contents_);
-  return nullptr;
+  return data;
 }
 
 MaybeLocal<Value> Shell::DeserializeValue(

src/d8.h

@@ -543,7 +543,7 @@ class Shell : public i::AllStatic {
   static base::LazyMutex context_mutex_;
   static const base::TimeTicks kInitialTicks;
 
-  static base::LazyMutex workers_mutex_;
+  static base::LazyMutex workers_mutex_;  // Guards the following members.
   static bool allow_new_workers_;
   static std::vector<Worker*> workers_;
   static std::vector<ExternalizedContents> externalized_contents_;