Update DataChannel bufferedamount implementation.

Call DataChannelObserver::OnBufferedAmountChange on each successful send.
Previously, the observer would get notified of buffered amount changes only when
queued send data is consumed. Data gets queued only if it cannot be sent right
away. According to the WebRTC standard[1], bufferedamount should be increased
before each sent and decreased after each successful sent. Update implementation
to be standard compliant.

Design doc: http://doc/1lorHBn-GMn5U0T0RQANxrsW0pXhw8XGZM-xZyVUOW90

[1] https://w3c.github.io/webrtc-pc/#dom-datachannel-bufferedamount

Bug: chromium:878682
Change-Id: Ife009d30c4a18dced9a54cf600a445bb1f02561d
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/123237
Commit-Queue: Marina Ciocea <marinaciocea@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#27057}

api/data_channel_interface.h

@@ -87,7 +87,7 @@ class DataChannelObserver {
   //  A data buffer was successfully received.
   virtual void OnMessage(const DataBuffer& buffer) = 0;
   // The data channel's buffered_amount has changed.
-  virtual void OnBufferedAmountChange(uint64_t previous_amount) {}
+  virtual void OnBufferedAmountChange(uint64_t sent_data_size) {}
 
  protected:
   virtual ~DataChannelObserver() = default;