Add UMA metric to track parallel download requests stats.

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 4bd781e1ebc7cd53937318d77f14e660fe177977..d0a1a4e34e6c443aedc5ebc440403a5288794f1b 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -13761,6 +13761,32 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Download.ParallelDownloadAddStreamSuccess"
+    enum="BooleanSuccess">
+  <owner>xingliu@chromium.org</owner>
+  <summary>
+    Records if the byte stream reader of a subsequent request is successfully
+    added to the download sink.
+  </summary>
+</histogram>
+
+<histogram name="Download.ParallelDownloadEarlyRequestCount">

[Ilya Sherman, 2017/03/31 00:33:04]

nit: Please add a units attribute.

[xingliu, 2017/03/31 21:48:27]

Done.

+  <owner>xingliu@chromium.org</owner>
+  <summary>
+    The total number of requests that have a smaller offset than the initial
+    request. This happens when the previous download is parallel and resume in
+    non-paralllel mode.
+  </summary>
+</histogram>
+
+<histogram name="Download.ParallelDownloadRequestCount">

[Ilya Sherman, 2017/03/31 00:33:04]

nit: Please add a units attribute.

[xingliu, 2017/03/31 21:48:27]

Done.

+  <owner>xingliu@chromium.org</owner>
+  <summary>
+    The total number of requests sent for a parallel download, including the
+    initial request.
+  </summary>
+</histogram>
+
 <histogram name="Download.PotentialBandwidth" units="Bytes/second">
   <obsolete>
     Deprecated January 2017.