Merged FeedbackSlotInterface into AstNode, removing the need for a 2nd vtable.

src/feedback-slots.h

-// Copyright 2014 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.
-
-#ifndef V8_FEEDBACK_SLOTS_H_
-#define V8_FEEDBACK_SLOTS_H_
-
-#include "src/v8.h"
-
-#include "src/isolate.h"
-
-namespace v8 {
-namespace internal {
-
-class FeedbackSlotInterface {
- public:
-  static const int kInvalidFeedbackSlot = -1;
-
-  virtual ~FeedbackSlotInterface() {}
-
-  virtual int ComputeFeedbackSlotCount() = 0;
-  virtual void SetFirstFeedbackSlot(int slot) = 0;
-};
-
-} }  // namespace v8::internal
-
-#endif  // V8_FEEDBACK_SLOTS_H_