Reland Use CopyOnWriteBuffer instead of Buffer to avoid unnecessary copies.

webrtc/base/buffer.cc

 /*
  *  Copyright 2015 The WebRTC 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 in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
 #include "webrtc/base/buffer.h"
 
 #include <algorithm>
 #include <utility>
 
 namespace rtc {
 
 Buffer::Buffer() : size_(0), capacity_(0), data_(nullptr) {
   RTC_DCHECK(IsConsistent());
 }
 
-Buffer::Buffer(const Buffer& buf) : Buffer(buf.data(), buf.size()) {
-}
-
 Buffer::Buffer(Buffer&& buf)
     : size_(buf.size()),
       capacity_(buf.capacity()),
       data_(std::move(buf.data_)) {
   RTC_DCHECK(IsConsistent());
   buf.OnMovedFrom();
 }
 
 Buffer::Buffer(size_t size) : Buffer(size, size) {
 }
 
 Buffer::Buffer(size_t size, size_t capacity)
     : size_(size),
       capacity_(std::max(size, capacity)),
       data_(new uint8_t[capacity_]) {
   RTC_DCHECK(IsConsistent());
 }
 
 // Note: The destructor works even if the buffer has been moved from.
 Buffer::~Buffer() = default;
 
 };  // namespace rtc