[v8] Allow querying the number of wrappers that still need to be traced

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 bea2dd88fdad33074e78e47d06414603b293616b..21409fc9668914973c57bc5a483c5534574902bc 100644
--- a/third_party/WebKit/Source/bindings/core/v8/ScriptWrappableVisitor.cpp
+++ b/third_party/WebKit/Source/bindings/core/v8/ScriptWrappableVisitor.cpp
@@ -55,6 +55,11 @@ void ScriptWrappableVisitor::AbortTracing()
     performCleanup();
 }
 
+size_t ScriptWrappableVisitor::NumberOfWrappersToTrace()
+{
+    return m_markingDeque.size();
+}
+
 void ScriptWrappableVisitor::performCleanup()
 {
     for (auto header : m_headersToUnmark) {