Set an expiry date for unused histograms (Download.*)

These are histograms that have not been viewed in the last 6 months, but still
report data on the Stable channel. Set these histograms to stop being recorded
for Chrome releases after 2018-08-30. We will file bugs via Monorail (crbug)
reminding owners about the approaching histogram expiry – initially 30 days
prior to expiry, and also with follow-up alerts. The metrics dashboards will
also display a warning for anyone viewing a soon-to-expire histogram.

Owners and/or users of these histograms: Please comment on this CL if you think
that any affected histogram should not have an expiry date set! This should be
uncommon.

Otherwise, there are a few options going forward:
  * If the histogram is no longer needed, please send a follow-up CL to delete
    the recording code and to mark the histogram as <obsolete>. (You can also
    remove the expiry date in the same CL.)
  * If you want to set a different expiry date, please send a follow-up CL to do
    so.
  * If neither of the above apply, then keep an eye out for Monorail bugs as the
    histogram expiry date approaches.
For more info on how unused histograms are identified, see
https://goto.google.com/uma-unused-histograms-cleanup

Bug: 850539
Change-Id: Ic01c7e6444cf45c2cc42bf58a6207bd3216b92f5
Reviewed-on: https://chromium-review.googlesource.com/1126400
Commit-Queue: Gayane Petrosyan <gayane@chromium.org>
Reviewed-by: Xing Liu <xingliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583033}

tools/metrics/histograms/histograms.xml

@@ -1628,7 +1628,7 @@ uploading your change for review.
 </histogram>
 
 <histogram name="Android.DownloadManager.ViewRetentionTime.Video"
-    units="minutes">
+    units="minutes" expires_after="2018-08-30">
   <owner>xingliu@chromium.org</owner>
   <summary>
     The duration between when a video file is downloaded and when the file is
@@ -21008,7 +21008,8 @@ uploading your change for review.
   </summary>
 </histogram>
 
-<histogram name="Download.DeleteRetentionTime.Video" units="hours">
+<histogram name="Download.DeleteRetentionTime.Video" units="hours"
+    expires_after="2018-08-30">
   <owner>xingliu@chromium.org</owner>
   <summary>
     How long users keep downloaded video files on disk. Recorded when the
@@ -21461,7 +21462,8 @@ uploading your change for review.
   </summary>
 </histogram>
 
-<histogram name="Download.ParallelDownload.BandwidthRatioPercentage" units="%">
+<histogram name="Download.ParallelDownload.BandwidthRatioPercentage" units="%"
+    expires_after="2018-08-30">
   <owner>xingliu@chromium.org</owner>
   <summary>
     Records the ratio of parallel streams bandwidth to single stream bandwidth
@@ -21490,7 +21492,8 @@ uploading your change for review.
   </summary>
 </histogram>
 
-<histogram name="Download.ParallelDownload.TotalTimeRatioPercentage" units="%">
+<histogram name="Download.ParallelDownload.TotalTimeRatioPercentage" units="%"
+    expires_after="2018-08-30">
   <owner>xingliu@chromium.org</owner>
   <summary>
     Records the ratio of the actual total time of the download to the total time
@@ -21500,7 +21503,7 @@ uploading your change for review.
 </histogram>
 
 <histogram name="Download.ParallelDownloadAddStreamSuccess"
-    enum="BooleanSuccess">
+    enum="BooleanSuccess" expires_after="2018-08-30">
   <owner>xingliu@chromium.org</owner>
   <summary>
     Records if the byte stream reader of a subsequent request is successfully
@@ -21508,7 +21511,8 @@ uploading your change for review.
   </summary>
 </histogram>
 
-<histogram name="Download.ParallelDownloadRequestCount" units="requests">
+<histogram name="Download.ParallelDownloadRequestCount" units="requests"
+    expires_after="2018-08-30">
   <owner>xingliu@chromium.org</owner>
   <summary>
     The total number of requests sent for a parallel download, including the