Reland Use CopyOnWriteBuffer instead of Buffer to avoid unnecessary copies.

webrtc/base/copyonwritebuffer.h

 /*
  *  Copyright 2016 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 24 matching lines @@
   CopyOnWriteBuffer(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>
   CopyOnWriteBuffer(const T* data, size_t size)
       : CopyOnWriteBuffer(data, size, size) {}
   template <typename T, typename internal::ByteType<T>::t = 0>
   CopyOnWriteBuffer(const T* data, size_t size, size_t capacity)
       : CopyOnWriteBuffer(size, capacity) {
-    std::memcpy(buffer_->data(), data, size);
+    if (buffer_) {
+      std::memcpy(buffer_->data(), data, size);
+    }
   }
 
   // Construct a buffer from the contents of an array.
   template <typename T, size_t N, typename internal::ByteType<T>::t = 0>
   CopyOnWriteBuffer(const T(&array)[N])  // NOLINT: runtime/explicit
       : CopyOnWriteBuffer(array, N) {}
 
   ~CopyOnWriteBuffer();
 
   // Get a pointer to the data. Just .data() will give you a (const) uint8_t*,
@@ skipping 60 matching lines @@
     RTC_DCHECK(buf.IsConsistent());
     return buffer_.get() == buf.buffer_.get() ||
         (buffer_.get() && buf.buffer_.get() &&
         *buffer_.get() == *buf.buffer_.get());
   }
 
   bool operator!=(const CopyOnWriteBuffer& buf) const {
     return !(*this == buf);
   }
 
+  uint8_t& operator[](size_t index) {
+    RTC_DCHECK_LT(index, size());
+    return data()[index];
+  }
+
+  uint8_t operator[](size_t index) const {
+    RTC_DCHECK_LT(index, size());
+    return cdata()[index];
+  }
+
   // Replace the contents of the buffer. Accepts the same types as the
   // constructors.
   template <typename T, typename internal::ByteType<T>::t = 0>
   void SetData(const T* data, size_t size) {
     RTC_DCHECK(IsConsistent());
     if (!buffer_ || !buffer_->HasOneRef()) {
-      buffer_ = new RefCountedObject<Buffer>(data, size, size);
+      buffer_ = size > 0 ? new RefCountedObject<Buffer>(data, size)
+                         : nullptr;
     } else {
       buffer_->SetData(data, size);
     }
     RTC_DCHECK(IsConsistent());
   }
 
   template <typename T, size_t N, typename internal::ByteType<T>::t = 0>
   void SetData(const T(&array)[N]) {
     SetData(array, N);
   }
@@ skipping 104 matching lines @@
     return (!buffer_ || buffer_->capacity() > 0);
   }
 
   // buffer_ is either null, or points to an rtc::Buffer with capacity > 0.
   scoped_refptr<RefCountedObject<Buffer>> buffer_;
 };
 
 }  // namespace rtc
 
 #endif  // WEBRTC_BASE_COPYONWRITEBUFFER_H_