Reland Use CopyOnWriteBuffer instead of Buffer to avoid unnecessary copies.

webrtc/base/buffer.h

 /*
  *  Copyright 2004 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.
  */
 
@@ skipping 31 matching lines @@
   using t = decltype(F(static_cast<T*>(nullptr)));
 };
 
 }  // namespace internal
 
 // Basic buffer class, can be grown and shrunk dynamically.
 // Unlike std::string/vector, does not initialize data when expanding capacity.
 class Buffer {
  public:
   Buffer();                   // An empty buffer.
-  Buffer(const Buffer& buf);  // Copy size and contents of an existing buffer.
   Buffer(Buffer&& buf);       // Move contents from an existing buffer.
 
   // Construct a buffer with the specified number of uninitialized bytes.
   explicit Buffer(size_t size);
   Buffer(size_t size, size_t capacity);
 
   // Construct a buffer and copy the specified number of bytes into it. The
   // source array may be (const) uint8_t*, int8_t*, or char*.
   template <typename T, typename internal::ByteType<T>::t = 0>
   Buffer(const T* data, size_t size)
@@ skipping 29 matching lines @@
   size_t size() const {
     RTC_DCHECK(IsConsistent());
     return size_;
   }
 
   size_t capacity() const {
     RTC_DCHECK(IsConsistent());
     return capacity_;
   }
 
-  Buffer& operator=(const Buffer& buf) {
-    if (&buf != this)
-      SetData(buf.data(), buf.size());
-    return *this;
-  }
-
   Buffer& operator=(Buffer&& buf) {
     RTC_DCHECK(IsConsistent());
     RTC_DCHECK(buf.IsConsistent());
     size_ = buf.size_;
     capacity_ = buf.capacity_;
     data_ = std::move(buf.data_);
     buf.OnMovedFrom();
     return *this;
   }
 
@@ skipping 145 matching lines @@
 #else
     // Ensure that *this is always inconsistent, to provoke bugs.
     size_ = 1;
     capacity_ = 0;
 #endif
   }
 
   size_t size_;
   size_t capacity_;
   std::unique_ptr<uint8_t[]> data_;
+
+  RTC_DISALLOW_COPY_AND_ASSIGN(Buffer);
 };
 
 }  // namespace rtc
 
 #endif  // WEBRTC_BASE_BUFFER_H_