Add UMA for studying download cancellation reasons

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 de559e729b416bdaba4d7ebfeb85eb59cb0fd32d..e6d8e30aab944dbed69f8cb6dfe6f8892bfad7cd 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -23281,6 +23281,11 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="MobileDownload.CancelReason" enum="MobileDownloadCancelReason">
+  <owner>qinmin@chromium.org</owner>
+  <summary>Android: Records the reason that a download is canceled.</summary>
+</histogram>
+
 <histogram name="MobileDownload.DownloadResumption"
     enum="MobileDownloadResumption">
   <owner>qinmin@chromium.org</owner>
@@ -80134,6 +80139,14 @@ To add a new entry, add it with any value and run test to compute valid value.
   <int value="1" label="Failure"/>
 </enum>
 
+<enum name="MobileDownloadCancelReason" type="int">
+  <int value="0" label="Not canceled"/>
+  <int value="1" label="Click on the cancel button"/>
+  <int value="2" label="Notification dismissed"/>
+  <int value="3" label="Overwrite infobar dismissed"/>
+  <int value="4" label="No storage permission"/>
+</enum>
+
 <enum name="MobileDownloadInterceptFailureReason" type="int">
   <int value="0" label="No failure"/>
   <int value="1" label="Empty url"/>