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 c309a7f5982044ec4d2c40b0c9ff24b8f1c21a95..21d401c17ce7b603673f47dde7044dbab7b4b23a 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -14149,6 +14149,14 @@ 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>
@@ -14170,6 +14178,15 @@ 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 total time of download with parallel streams to
+    actual total time. The total time with parallel streams is calculated as the
+    size of the file divided by parallel streams bandwidth.
+  </summary>
+</histogram>
+
 <histogram name="Download.ParallelDownloadAddStreamSuccess"
     enum="BooleanSuccess">
   <owner>xingliu@chromium.org</owner>