Introduce infrastructure for tracing ScriptWrappables.

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

+// 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.
+
+#ifndef ScriptWrappableHeapTracer_h
+#define ScriptWrappableHeapTracer_h
+
+#include <v8.h>
+#include <vector>
+
+namespace blink {
+
+class HeapObjectHeader;
+
+/**
+ * When this heap tracer is set to the v8::Isolate, the V8 will call it during
+ * its garbage collection. At the beginning, it will call TraceRoots, then
+ * repeatedly (as v8 discovers more wrappers) it will call TraceWrappableFrom,
+ * and at the end it will call ClearTracingMarks.
+ */
+class ScriptWrappableHeapTracer : public v8::EmbedderHeapTracer {
+public:
+    ~ScriptWrappableHeapTracer() override;
+    void TraceRoots(v8::Isolate*) override;
+    void TraceWrappableFrom(v8::Isolate*, const std::vector<std::pair<void*, void*>>& hiddenFieldsOfPotentialWrappers) override;
+    void TraceWrappableFrom(v8::Isolate*, v8::Persistent<v8::Object>* wrapper);
+    void AddHeaderToUnmark(HeapObjectHeader*);
+    void ClearTracingMarks(v8::Isolate*) override;
+private:
+    void TraceWrappableFrom(v8::Isolate*, std::pair<void*, void*> hiddenFields);
+    bool m_tracingInProgress = false;
+    std::vector<HeapObjectHeader*> m_headersToUnmark;
+};
+
+} // namespace blink
+
+#endif // ScriptWrappableHeapTracer_h