calculateThroughputByChunkData() fixes (#4130)

* Add reset of shortDurationStartTime after chunkThroughputs.push()

- shortDurationStartTime need to be reset after a
  chunkThroughputs.push() otherwise it can lead to spurious
  'short duration' throughput measurements.
- Moved reset of shortDurationBytesReceived to more optimal location so
  it is only reset when shortDurationStartTime === 0

* Included length of last piece of short duration data in throughput calculation

- The length (byte count) of the final piece of 'short duration'
  data should be included in the throughput calculation otherwise
  it is ignored which leads to an underestimate.

src/streaming/net/FetchLoader.js

@@ -426,14 +426,16 @@ function FetchLoader(cfg) {
                         let chunkDownloadTime = datumE[i].ts - datum[i].ts;
                         if (chunkDownloadTime > 1) {
                             chunkThroughputs.push((8 * datumE[i].bytes) / chunkDownloadTime);
+                            shortDurationStartTime = 0;
                         } else {
                             if (shortDurationStartTime === 0) {
                                 shortDurationStartTime = datum[i].ts;
+                                shortDurationBytesReceived = 0;
                             }
                             let cumulatedChunkDownloadTime = datumE[i].ts - shortDurationStartTime;
                             if (cumulatedChunkDownloadTime > 1) {
+                                shortDurationBytesReceived += datumE[i].bytes;
                                 chunkThroughputs.push((8 * shortDurationBytesReceived) / cumulatedChunkDownloadTime);
-                                shortDurationBytesReceived = 0;
                                 shortDurationStartTime = 0;
                             } else {
                                 // continue cumulating short duration data