Set an expiry date for unused histograms.

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: I5bb941b74e051648a8863bcc6922f3c43b6eb51f
Reviewed-on: https://chromium-review.googlesource.com/1126337Reviewed-by: Taiju Tsuiki <tzik@chromium.org>
Commit-Queue: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#573511}

tools/metrics/histograms/histograms.xml

@@ -30619,7 +30619,7 @@ uploading your change for review.
   </summary>
 </histogram>
 
-<histogram name="FileSystem.PersistentOriginsCount">
+<histogram name="FileSystem.PersistentOriginsCount" expires_after="2018-08-30">
   <owner>tzik@chromium.org</owner>
   <summary>
     Number of origins that have persistent filesystem. Measured when the Quota
@@ -30629,7 +30629,7 @@ uploading your change for review.
   </summary>
 </histogram>
 
-<histogram name="FileSystem.TemporaryOriginsCount">
+<histogram name="FileSystem.TemporaryOriginsCount" expires_after="2018-08-30">
   <owner>tzik@chromium.org</owner>
   <summary>
     Number of origins that have temporary filesystem. Measured when the Quota
@@ -78372,7 +78372,8 @@ uploading your change for review.
   </summary>
 </histogram>
 
-<histogram name="Quota.ErrorsOnEvictingOriginPerHour">
+<histogram name="Quota.ErrorsOnEvictingOriginPerHour"
+    expires_after="2018-08-30">
   <owner>tzik@chromium.org</owner>
   <summary>
     Number of errors on evicting origin by QuotaTemporaryStorageEvictor in an
@@ -78380,7 +78381,8 @@ uploading your change for review.
   </summary>
 </histogram>
 
-<histogram name="Quota.ErrorsOnGettingUsageAndQuotaPerHour">
+<histogram name="Quota.ErrorsOnGettingUsageAndQuotaPerHour"
+    expires_after="2018-08-30">
   <owner>tzik@chromium.org</owner>
   <summary>
     Number of errors on getting usage and quota by QuotaTemporaryStorageEvictor
@@ -78473,32 +78475,38 @@ uploading your change for review.
   <summary>Number of evicted origins per round.</summary>
 </histogram>
 
-<histogram name="Quota.NumberOfPersistentStorageOrigins">
+<histogram name="Quota.NumberOfPersistentStorageOrigins"
+    expires_after="2018-08-30">
   <owner>tzik@chromium.org</owner>
   <summary>Number of origins using persistent storage.</summary>
 </histogram>
 
-<histogram name="Quota.NumberOfProtectedPersistentStorageOrigins">
+<histogram name="Quota.NumberOfProtectedPersistentStorageOrigins"
+    expires_after="2018-08-30">
   <owner>tzik@chromium.org</owner>
   <summary>Number of protected origins using persistent storage.</summary>
 </histogram>
 
-<histogram name="Quota.NumberOfProtectedTemporaryStorageOrigins">
+<histogram name="Quota.NumberOfProtectedTemporaryStorageOrigins"
+    expires_after="2018-08-30">
   <owner>tzik@chromium.org</owner>
   <summary>Number of protected origins using temporary storage.</summary>
 </histogram>
 
-<histogram name="Quota.NumberOfTemporaryStorageOrigins">
+<histogram name="Quota.NumberOfTemporaryStorageOrigins"
+    expires_after="2018-08-30">
   <owner>tzik@chromium.org</owner>
   <summary>Number of origins using temporary storage.</summary>
 </histogram>
 
-<histogram name="Quota.NumberOfUnlimitedPersistentStorageOrigins">
+<histogram name="Quota.NumberOfUnlimitedPersistentStorageOrigins"
+    expires_after="2018-08-30">
   <owner>tzik@chromium.org</owner>
   <summary>Number of unlimited origins using persistent storage.</summary>
 </histogram>
 
-<histogram name="Quota.NumberOfUnlimitedTemporaryStorageOrigins">
+<histogram name="Quota.NumberOfUnlimitedTemporaryStorageOrigins"
+    expires_after="2018-08-30">
   <owner>tzik@chromium.org</owner>
   <summary>Number of unlimited origins using temporary storage.</summary>
 </histogram>
@@ -78549,7 +78557,7 @@ uploading your change for review.
   </summary>
 </histogram>
 
-<histogram name="Quota.SkippedEvictionRoundsPerHour">
+<histogram name="Quota.SkippedEvictionRoundsPerHour" expires_after="2018-08-30">
   <owner>tzik@chromium.org</owner>
   <summary>Number of skipped eviction rounds in an hour.</summary>
 </histogram>
@@ -78568,7 +78576,7 @@ uploading your change for review.
   <summary>Time between two consecutive active eviction rounds.</summary>
 </histogram>
 
-<histogram name="Quota.TimeSpentToAEvictionRound">
+<histogram name="Quota.TimeSpentToAEvictionRound" expires_after="2018-08-30">
   <owner>tzik@chromium.org</owner>
   <summary>Time spent to an eviction round.</summary>
 </histogram>
@@ -78599,7 +78607,8 @@ uploading your change for review.
   </summary>
 </histogram>
 
-<histogram name="Quota.UsageOverageOfTemporaryGlobalStorage" units="MB">
+<histogram name="Quota.UsageOverageOfTemporaryGlobalStorage" units="MB"
+    expires_after="2018-08-30">
   <owner>tzik@chromium.org</owner>
   <summary>
     Overage of the temporary global storage usage at beginning of an eviction
@@ -100082,7 +100091,7 @@ uploading your change for review.
 </histogram>
 
 <histogram name="SyncFileSystem.RegisterOriginResult"
-    enum="SyncFSRemoteServiceState">
+    enum="SyncFSRemoteServiceState" expires_after="2018-08-30">
   <owner>tzik@chromium.org</owner>
   <summary>
     The result of the registration of Chrome App to Sync FileSystem.
@@ -100098,7 +100107,8 @@ uploading your change for review.
   </summary>
 </histogram>
 
-<histogram name="SyncFileSystem.TrackerDB.Open" enum="LevelDBStatus">
+<histogram name="SyncFileSystem.TrackerDB.Open" enum="LevelDBStatus"
+    expires_after="2018-08-30">
   <owner>tzik@chromium.org</owner>
   <summary>The result of opening the Sync FileSystem tracker database.</summary>
 </histogram>