Revert of Pool implementation for zone segments

src/zone/zone-segment.h

 // Copyright 2016 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_ZONE_ZONE_SEGMENT_H_
 #define V8_ZONE_ZONE_SEGMENT_H_
 
 #include "src/v8.h"
 
 // Segments represent chunks of memory: They have starting address
@@ skipping 20 matching lines @@
 
   Segment* next() const { return next_; }
   void set_next(Segment* const next) { next_ = next; }
 
   size_t size() const { return size_; }
   size_t capacity() const { return size_ - sizeof(Segment); }
 
   Address start() const { return address(sizeof(Segment)); }
   Address end() const { return address(size_); }
 
-  // Zap the contents of the segment (but not the header).
-  void ZapContents();
-  // Zaps the header and makes the segment unusable this way.
-  void ZapHeader();
-
  private:
-#ifdef DEBUG
-  // Constant byte value used for zapping dead memory in debug mode.
-  static const unsigned char kZapDeadByte = 0xcd;
-#endif
-
   // Computes the address of the nth byte in this segment.
   Address address(size_t n) const { return Address(this) + n; }
 
   Zone* zone_;
   Segment* next_;
   size_t size_;
 };
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_ZONE_ZONE_SEGMENT_H_