Reland Use CopyOnWriteBuffer instead of Buffer to avoid unnecessary copies.

talk/app/webrtc/objc/RTCDataChannel.mm

 /*
  * libjingle
  * Copyright 2014 Google Inc.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are met:
  *
  *  1. Redistributions of source code must retain the above copyright notice,
  *     this list of conditions and the following disclaimer.
  *  2. Redistributions in binary form must reproduce the above copyright notice,
@@ skipping 135 matching lines @@
 
 @end
 
 @implementation RTCDataBuffer {
   std::unique_ptr<webrtc::DataBuffer> _dataBuffer;
 }
 
 - (instancetype)initWithData:(NSData*)data isBinary:(BOOL)isBinary {
   NSAssert(data, @"data cannot be nil");
   if (self = [super init]) {
-    rtc::Buffer buffer(reinterpret_cast<const uint8_t*>([data bytes]),
-                       [data length]);
+    rtc::CopyOnWriteBuffer buffer(
+        reinterpret_cast<const uint8_t*>([data bytes]), [data length]);
     _dataBuffer.reset(new webrtc::DataBuffer(buffer, isBinary));
   }
   return self;
 }
 
 - (NSData*)data {
   return [NSData dataWithBytes:_dataBuffer->data.data()
                         length:_dataBuffer->data.size()];
 }
 
@@ skipping 115 matching lines @@
     _observer.reset(new webrtc::RTCDataChannelObserver(self));
   }
   return self;
 }
 
 - (rtc::scoped_refptr<webrtc::DataChannelInterface>)dataChannel {
   return _dataChannel;
 }
 
 @end