Add the metric to record percentage improvement for 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 c2a2f5bfc680bbdc9249e41fd593e81938b36af9..172c1a3ec7368464e95fe54b3dccfd0b34811219 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -14213,6 +14213,14 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Download.ParallelDownload.BandwidthRatioPercentage" units="%">
+  <owner>xingliu@chromium.org</owner>
+  <summary>
+    Records the ratio of parallel streams bandwidth to single stream bandwidth
+    in parallel download.
+  </summary>
+</histogram>
+
 <histogram name="Download.ParallelDownload.CreationEvent"
     enum="ParallelDownloadCreationEvent">
   <owner>xingliu@chromium.org</owner>
@@ -14234,6 +14242,15 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Download.ParallelDownload.TotalTimeRatioPercentage" units="%">
+  <owner>xingliu@chromium.org</owner>
+  <summary>
+    Records the ratio of the actual total time of the download to the total time
+    without parallel streams. The total time without parallel streams is
+    calculated as the size of the file divided by the single stream bandwidth.
+  </summary>
+</histogram>
+
 <histogram name="Download.ParallelDownloadAddStreamSuccess"
     enum="BooleanSuccess">
   <owner>xingliu@chromium.org</owner>