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

BUG=chromium:468240

Review-Url: https://codereview.chromium.org/2288253003
Cr-Commit-Position: refs/heads/master@{#415281}

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) {

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

@@ -101,6 +101,7 @@ public:
     void TraceEpilogue() override;
     void AbortTracing() override;
     void EnterFinalPause() override;
+    size_t NumberOfWrappersToTrace() override;
 
     void dispatchTraceWrappers(const ScriptWrappable*) const override;
 #define DECLARE_DISPATCH_TRACE_WRAPPERS(className)                   \