Introduce api for incremental wrapper tracing

include/v8.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 /** \mainpage V8 API Reference Guide
  *
  * V8 is Google's open source JavaScript engine.
  *
  * This set of documents provides reference material generated from the
  * V8 header file, include/v8.h.
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  * end of its marking phase iterates the collection and asks the embedder to
  * trace through its heap and call PersistentBase::RegisterExternalReference on
  * each js object reachable from any of the given wrappers.
  *
  * Before the first call to the TraceWrappersFrom function TracePrologue will be
  * called. When the garbage collection cycle is finished, TraceEpilogue will be
  * called.
  */
 class V8_EXPORT EmbedderHeapTracer {
  public:
+  enum ForceCompletionAction { FORCE_COMPLETION, DO_NOT_FORCE_COMPLETION };
+  struct AdvanceTracingActions {
+    explicit AdvanceTracingActions(ForceCompletionAction force_completion_)
+        : force_completion(force_completion_) {}
+
+    ForceCompletionAction force_completion;
+  };
+  /**
+   * V8 will call this method with internal fields of found wrappers.
+   * Embedder is expected to store them in it's marking deque and trace
+   * reachable wrappers from them when asked by AdvanceTracing method.
+   */
+  // TODO(hlopko): Make pure virtual after migration
+  virtual void RegisterV8References(
+      const std::vector<std::pair<void*, void*> >& internal_fields) {}
+  /**
+   * **Deprecated**
+   */
+  // TODO(hlopko): Remove after migration
+  virtual void TraceWrappersFrom(
+      const std::vector<std::pair<void*, void*> >& internal_fields) {}
   /**
    * V8 will call this method at the beginning of the gc cycle.
    */
   virtual void TracePrologue() = 0;
   /**
-   * V8 will call this method with internal fields of a potential wrappers.
-   * Embedder is expected to trace its heap (synchronously) and call
-   * PersistentBase::RegisterExternalReference() on all wrappers reachable from
-   * any of the given wrappers.
+   * Embedder is expected to trace its heap starting from wrappers reported by
+   * RegisterV8References method, and call
+   * PersistentBase::RegisterExternalReference() on all reachable wrappers.
+   * Embedder is expected to stop tracing by the given deadline.
+   *
+   * Returns true if there is still work to do.
    */
-  virtual void TraceWrappersFrom(
-      const std::vector<std::pair<void*, void*> >& internal_fields) = 0;
+  // TODO(hlopko): Make pure virtual after migration
+  virtual bool AdvanceTracing(double deadline_in_ms,
+                              AdvanceTracingActions actions) {
+    return false;
+  }
   /**
    * V8 will call this method at the end of the gc cycle. Allocation is *not*
    * allowed in the TraceEpilogue.
    */
   virtual void TraceEpilogue() = 0;
 
  protected:
   virtual ~EmbedderHeapTracer() = default;
 };
 
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  */
 
 
 }  // namespace v8
 
 
 #undef TYPE_CHECK
 
 
 #endif  // INCLUDE_V8_H_