Add UMA for estimating disk bandwidth and the time saved with parallel download

tools/metrics/histograms/histograms.xml

Index: tools/metrics/histograms/histograms.xml
diff --git a/tools/metrics/histograms/histograms.xml b/tools/metrics/histograms/histograms.xml
index baeafb30b2ed6acc17dc513735c857883a2932b8..625559ba62e3b377e9e02d16f0b2456c49a74150 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -13036,6 +13036,27 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Download.BandwidthWithoutParallelStreamsBytesPerSecond"
+    units="bytes/second">
+  <owner>qinmin@chromium.org</owner>
+  <summary>
+    For parallel downloading, average disk bandwidth seen when only 1 stream is
+    actively downloading the content. Other streams may have already finished,
+    or have not been created yet. If a download was ever paused, this is not
+    recorded.
+  </summary>
+</histogram>
+
+<histogram name="Download.BandwidthWithParallelStreamsBytesPerSecond"
+    units="bytes/second">
+  <owner>qinmin@chromium.org</owner>
+  <summary>
+    For parallel downloading, average disk bandwidth seen when parallel streams
+    are downloading the content. If a download was ever paused, this is not
+    recorded.
+  </summary>
+</histogram>
+
 <histogram name="Download.ClearAllSize">
   <obsolete>
     Deprecated 1/2017.
@@ -13243,6 +13264,29 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Download.EstimatedTimeSavedWithParallelDownload" units="ms">
+  <owner>qinmin@chromium.org</owner>
+  <summary>
+    Estimated time saved on a download when parallel downloading is enabled. To
+    estimate this, the time spent on a download is classified into two
+    categories. The fist category has only one stream active, while other
+    streams are either finished, not started, or failed. And the second category
+    has multiple streams active. By calculating the average bandwidth during the
+    first category, a rough estimation on the time it will save is obtained by
+    assuming all the bytes downloaded during the second category are downloaded
+    by only one stream. If a download is ever paused, this is not recorded.
+  </summary>
+</histogram>
+
+<histogram name="Download.EstimatedTimeWastedWithParallelDownload" units="ms">
+  <owner>qinmin@chromium.org</owner>
+  <summary>
+    Similar to Download.EstimatedTimeSavedWithParallelDownload, but this
+    estimates the time wasted on a download when parallel downloading is
+    enabled.
+  </summary>
+</histogram>
+
 <histogram name="Download.FeedbackDialogEnabled" enum="BooleanEnabled">
   <owner>asanka@chromium.org</owner>
   <summary>