Revert of Revert "[wrapper-tracing] Bail out tracing for empty handles"

third_party/WebKit/Source/bindings/core/v8/ScriptWrappableVisitor.cpp

 // Copyright 2016 The Chromium 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 "bindings/core/v8/ScriptWrappableVisitor.h"
 
 #include "bindings/core/v8/ActiveScriptWrappable.h"
 #include "bindings/core/v8/DOMWrapperWorld.h"
 #include "bindings/core/v8/ScopedPersistent.h"
 #include "bindings/core/v8/ScriptWrappable.h"
@@ skipping 240 matching lines @@
 
 void ScriptWrappableVisitor::TraceWrappers(
     const TraceWrapperV8Reference<v8::Value>& traced_wrapper) const {
   MarkWrapper(
       &(const_cast<TraceWrapperV8Reference<v8::Value>&>(traced_wrapper).Get()));
 }
 
 void ScriptWrappableVisitor::MarkWrapper(
     const v8::PersistentBase<v8::Value>* handle) const {
   // The write barrier may try to mark a wrapper because cleanup is still
-  // delayed. Bail out in this case.
-  if (!m_tracingInProgress)
+  // delayed. Bail out in this case. We also allow unconditional marking which
+  // requires us to bail out here when tracing is not in progress.
+  if (!tracing_in_progress_ || handle->IsEmpty())
     return;
   handle->RegisterExternalReference(isolate_);
 }
 
 void ScriptWrappableVisitor::DispatchTraceWrappers(
     const TraceWrapperBase* wrapper_base) const {
   wrapper_base->TraceWrappers(this);
 }
 
 void ScriptWrappableVisitor::InvalidateDeadObjectsInMarkingDeque() {
@@ skipping 31 matching lines @@
       V8PerIsolateData::From(isolate)->GetScriptWrappableVisitor();
   if (script_wrappable_visitor)
     script_wrappable_visitor->PerformCleanup();
 }
 
 WrapperVisitor* ScriptWrappableVisitor::CurrentVisitor(v8::Isolate* isolate) {
   return V8PerIsolateData::From(isolate)->GetScriptWrappableVisitor();
 }
 
 }  // namespace blink