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 26c5bc4c1ea47cb99c05273ac7d6415af05f9441..bdc61910caca434d14f79760f16fd982fef0fe3f 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -13036,6 +13036,25 @@ 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.
+  </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.
+  </summary>
+</histogram>
+
 <histogram name="Download.ClearAllSize">
   <obsolete>
     Deprecated 1/2017.
@@ -13598,6 +13617,15 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <summary>Time between the start of a download and its completion.</summary>
 </histogram>
 
+<histogram name="Download.TimeSavedWithParallelDownload" units="ms">

[David Trainor- moved to gerrit, 2017/03/14 15:28:38]

Can we describe how we expect these to work over the course of a download (e.g.
it looks like the last stream will be counted as a single stream if the others
have finished, etc.)?

[qinmin, 2017/03/15 00:17:19]

Done.

+  <owner>qinmin@chromium.org</owner>
+  <summary>
+    Estimated time saved on a download when parallel downloading is enabled. The
+    value could be nagative if using parallel requests is slower than using a
+    single request.
+  </summary>
+</histogram>
+
 <histogram name="Download.TimeToRenameFailureAfterInitialFailure" units="ms">
   <owner>asanka@chromium.org</owner>
   <summary>