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

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

Index: third_party/WebKit/Source/bindings/core/v8/ScriptWrappableVisitor.cpp
diff --git a/third_party/WebKit/Source/bindings/core/v8/ScriptWrappableVisitor.cpp b/third_party/WebKit/Source/bindings/core/v8/ScriptWrappableVisitor.cpp
index 4fa05adc46eba6575e978539639cb1299d3d0fd2..eac79f1e60c404c9a5bbb6ef7c1532b80321a4e9 100644
--- a/third_party/WebKit/Source/bindings/core/v8/ScriptWrappableVisitor.cpp
+++ b/third_party/WebKit/Source/bindings/core/v8/ScriptWrappableVisitor.cpp
@@ -258,8 +258,9 @@
 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_);
 }