Reland Use CopyOnWriteBuffer instead of Buffer to avoid unnecessary copies.

webrtc/api/objc/RTCDataChannel.mm

Index: webrtc/api/objc/RTCDataChannel.mm
diff --git a/webrtc/api/objc/RTCDataChannel.mm b/webrtc/api/objc/RTCDataChannel.mm
index 5778cb5b7afd19602c754c6467268cbd0c6c06a2..98599058ee8ebec28ba52f096ad3a816c56b8034 100644
--- a/webrtc/api/objc/RTCDataChannel.mm
+++ b/webrtc/api/objc/RTCDataChannel.mm
@@ -53,8 +53,8 @@ class DataChannelDelegateAdapter : public DataChannelObserver {
 - (instancetype)initWithData:(NSData *)data isBinary:(BOOL)isBinary {
   NSParameterAssert(data);
   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;