Deserialize the scope chain for background parsing jobs on the main thread

Even though there is no scope chain, just the empty outer scope.

R=marja@chromium.org
BUG=

Review-Url: https://codereview.chromium.org/2273553003
Cr-Commit-Position: refs/heads/master@{#38814}

src/background-parsing-task.cc

@@ -42,6 +42,9 @@ BackgroundParsingTask::BackgroundParsingTask(
   // Parser needs to stay alive for finalizing the parsing on the main
   // thread.
   source_->parser.reset(new Parser(source_->info.get()));
+  source_->parser->DeserializeScopeChain(
+      source_->info.get(), Handle<Context>::null(),
+      Scope::DeserializationMode::kDeserializeOffHeap);
 }
 
 
@@ -61,9 +64,6 @@ void BackgroundParsingTask::Run() {
   Isolate* isolate = source_->info->isolate();
   source_->info->set_isolate(nullptr);
 
-  source_->parser->DeserializeScopeChain(
-      source_->info.get(), Handle<Context>::null(),
-      Scope::DeserializationMode::kDeserializeOffHeap);
   source_->parser->ParseOnBackground(source_->info.get());
 
   if (script_data_ != nullptr) {

src/parsing/parser.cc

@@ -767,6 +767,7 @@ Parser::Parser(ParseInfo* info)
 void Parser::DeserializeScopeChain(
     ParseInfo* info, Handle<Context> context,
     Scope::DeserializationMode deserialization_mode) {
+  DCHECK(ThreadId::Current().Equals(info->isolate()->thread_id()));
   // TODO(wingo): Add an outer SCRIPT_SCOPE corresponding to the native
   // context, which will have the "this" binding for script scopes.
   DeclarationScope* script_scope = NewScriptScope();