Reland Use CopyOnWriteBuffer instead of Buffer to avoid unnecessary copies.

talk/app/webrtc/objc/RTCDataChannel.mm

Index: talk/app/webrtc/objc/RTCDataChannel.mm
diff --git a/talk/app/webrtc/objc/RTCDataChannel.mm b/talk/app/webrtc/objc/RTCDataChannel.mm
index 114b916fb467e1c7255464242853e5783d83e48e..6cfa5623c4b3df572405eb2e48789a802c92c7fe 100644
--- a/talk/app/webrtc/objc/RTCDataChannel.mm
+++ b/talk/app/webrtc/objc/RTCDataChannel.mm
@@ -153,8 +153,8 @@ std::string StdStringFromNSString(NSString* nsString) {
 - (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;