Trace active script wrappables on entering final pause

V8 now tells EmbedderHeapTracer when it enters the final pause, and that is
exactly the place where we need to trace active script wrappables. Previous
implementation is not correct.

LOG=no
BUG=468240

Review-Url: https://codereview.chromium.org/2076133002
Cr-Commit-Position: refs/heads/master@{#400634}

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

@@ -27,11 +27,23 @@ void ScriptWrappableVisitor::TracePrologue()
     m_tracingInProgress = true;
 }
 
-void ScriptWrappableVisitor::TraceEpilogue()
+void ScriptWrappableVisitor::EnterFinalPause()
 {
     ActiveScriptWrappable::traceActiveScriptWrappables(m_isolate, this);
-    AdvanceTracing(0, AdvanceTracingActions(ForceCompletionAction::FORCE_COMPLETION));
+}
+
+void ScriptWrappableVisitor::TraceEpilogue()
+{
+    performCleanup();
+}
 
+void ScriptWrappableVisitor::AbortTracing()
+{
+    performCleanup();
+}
+
+void ScriptWrappableVisitor::performCleanup()
+{
     for (auto header : m_headersToUnmark) {
         header->unmarkWrapperHeader();
     }

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

@@ -57,6 +57,8 @@ public:
     void RegisterV8Reference(const std::pair<void*, void*>& internalFields);
     bool AdvanceTracing(double deadlineInMs, v8::EmbedderHeapTracer::AdvanceTracingActions) override;
     void TraceEpilogue() override;
+    void AbortTracing() override;
+    void EnterFinalPause() override;
 
     template <typename T>
     void markWrapper(const v8::Persistent<T>* handle) const
@@ -82,6 +84,7 @@ private:
     bool markWrapperHeader(const ScriptWrappable*) const override;
     bool markWrapperHeader(const void* garbageCollected) const override;
     bool m_tracingInProgress = false;
+    void performCleanup();
     /**
      * Collection of ScriptWrappables we need to trace from. We assume it is
      * safe to hold on to the raw pointers because: