Use rtc::CopyOnWriteBuffer::MutableData instead of data

mutable data() function will be removed to avoid unintended underlying memcpy.

Bug: webrtc:12334
Change-Id: Iab9ab9311db2132011b5a21a2b230ed3314aad88
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2620240
Commit-Queue: Yuwei Huang <yuweih@chromium.org>
Auto-Submit: Danil Chapovalov <danilchap@chromium.org>
Reviewed-by: Yuwei Huang <yuweih@chromium.org>
Cr-Commit-Position: refs/heads/master@{#842204}

remoting/protocol/webrtc_data_stream_adapter.cc

@@ -59,8 +59,7 @@ void WebrtcDataStreamAdapter::Send(google::protobuf::MessageLite* message,
 
   rtc::CopyOnWriteBuffer buffer;
   buffer.SetSize(message->ByteSize());
-  message->SerializeWithCachedSizesToArray(
-      reinterpret_cast<uint8_t*>(buffer.data()));
+  message->SerializeWithCachedSizesToArray(buffer.MutableData());
   pending_messages_.emplace(
       webrtc::DataBuffer(std::move(buffer), true /* binary */),
       std::move(done));