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 790748cfab6013c941832b528f865b61ac5721bc..80f090fdaea2e71b0be5a31500e6689e82db3b0a 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -13865,6 +13865,14 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Download.ParallelDownloadFailReason"

[qinmin, 2017/04/07 21:00:52]

Download.ParallelDownload.CreationFailureReason?

[xingliu, 2017/04/08 01:13:28]

Done.

+    enum="ParallelDownloadFailReason">
+  <owner>xingliu@chromium.org</owner>
+  <summary>
+    Records why the download is not created as parallel download.

[Ilya Sherman, 2017/04/07 22:13:08]

Please document when this metric is recorded.  Is it on download start, for
every download, unless the download was created as a parallel download?  Is
there any pre-filtering?

[Ilya Sherman, 2017/04/07 22:13:09]

I'd recommend including "success" as bucket zero in this histogram, so that you
can see the overall failure rate as well.  (If you make this change, you'll
likely want to also rename the histogram and update the description.)

+  </summary>
+</histogram>
+
 <histogram name="Download.ParallelDownloadRequestCount" units="requests">
   <owner>xingliu@chromium.org</owner>
   <summary>
@@ -105714,6 +105722,19 @@ from previous Chrome versions.
   <int value="1" label="HRTF"/>
 </enum>
 
+<enum name="ParallelDownloadFailReason" type="int">
+  <summary>
+    The reason why parallel download is not applicable and fallbacks to a
+    non-parallel download.
+  </summary>
+  <int value="0" label="STRONG_VALIDATORS"/>
+  <int value="1" label="ACCEPT_RANGE_HEADER"/>
+  <int value="2" label="CONTENT_LENGTH_HEADER"/>
+  <int value="3" label="FILE_SIZE"/>
+  <int value="4" label="CONNECTION_TYPE"/>
+  <int value="5" label="FAIL_COUNT"/>
+</enum>
+
 <enum name="ParsedCookieStatus" type="int">
   <obsolete>
     Deprecated as of 9/2013. Experiment to measure control characters in cookies