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: I437ea5c000a3a4fe3a065dd51641de44a27a6fa9
Reviewed-on: https://chromium-review.googlesource.com/1126321
Commit-Queue: Gayane Petrosyan <gayane@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584894}

tools/metrics/histograms/histograms.xml

@@ -27468,7 +27468,7 @@ uploading your change for review.
 </histogram>
 
 <histogram name="ExtensionInstalledLoader.ForceDisabled2"
-    enum="BooleanForceDisabled">
+    enum="BooleanForceDisabled" expires_after="2018-08-30">
   <owner>rdevlin.cronin@chromium.org</owner>
   <summary>
     Counts whether we force-disabled an installed extension at startup because
@@ -27814,7 +27814,8 @@ uploading your change for review.
   </summary>
 </histogram>
 
-<histogram name="Extensions.BackgroundContentsServiceStartupTime" units="ms">
+<histogram name="Extensions.BackgroundContentsServiceStartupTime" units="ms"
+    expires_after="2018-08-30">
   <owner>rdevlin.cronin@chromium.org</owner>
   <summary>
     Time taken to load BackgroundContents for apps at startup when the extension
@@ -28581,7 +28582,7 @@ uploading your change for review.
 </histogram>
 
 <histogram name="Extensions.DidSuppressJavaScriptException"
-    enum="ChromeChannelForHistogram">
+    enum="ChromeChannelForHistogram" expires_after="2018-08-30">
   <owner>rdevlin.cronin@chromium.org</owner>
   <summary>
     Records the Chrome channel when JavaScript exceptions are not gracefully
@@ -28856,7 +28857,8 @@ uploading your change for review.
   </summary>
 </histogram>
 
-<histogram name="Extensions.ExtensionHostMonitoring.MaxActiveLoading">
+<histogram name="Extensions.ExtensionHostMonitoring.MaxActiveLoading"
+    expires_after="2019-12-31">
   <owner>rdevlin.cronin@chromium.org</owner>
   <summary>
     The maximum number of ExtensionHosts (primarily background/event pages) that
@@ -28865,7 +28867,8 @@ uploading your change for review.
   </summary>
 </histogram>
 
-<histogram name="Extensions.ExtensionHostMonitoring.MaxInQueue">
+<histogram name="Extensions.ExtensionHostMonitoring.MaxInQueue"
+    expires_after="2019-12-31">
   <owner>rdevlin.cronin@chromium.org</owner>
   <summary>
     The maximum number of ExtensionHosts (primarily background/event pages) in
@@ -28874,7 +28877,8 @@ uploading your change for review.
   </summary>
 </histogram>
 
-<histogram name="Extensions.ExtensionHostMonitoring.NumLoaded">
+<histogram name="Extensions.ExtensionHostMonitoring.NumLoaded"
+    expires_after="2019-12-31">
   <owner>rdevlin.cronin@chromium.org</owner>
   <summary>
     The total number of ExtensionHosts that completed loading in the first
@@ -28882,7 +28886,8 @@ uploading your change for review.
   </summary>
 </histogram>
 
-<histogram name="Extensions.ExtensionHostMonitoring.NumQueued">
+<histogram name="Extensions.ExtensionHostMonitoring.NumQueued"
+    expires_after="2019-12-31">
   <owner>rdevlin.cronin@chromium.org</owner>
   <summary>
     The total number of ExtensionHosts (primarily background/event pages) that
@@ -28940,7 +28945,8 @@ uploading your change for review.
   </summary>
 </histogram>
 
-<histogram name="Extensions.ExtensionServiceNotifyReadyListenersTime">
+<histogram name="Extensions.ExtensionServiceNotifyReadyListenersTime"
+    expires_after="2019-12-31">
   <owner>rdevlin.cronin@chromium.org</owner>
   <summary>
     Time taken for the ExtensionService to notify all ready listeners that the
@@ -29016,7 +29022,8 @@ uploading your change for review.
   </summary>
 </histogram>
 
-<histogram name="Extensions.ExternalWarningUninstallationResult" enum="Boolean">
+<histogram name="Extensions.ExternalWarningUninstallationResult" enum="Boolean"
+    expires_after="2018-08-30">
   <owner>rdevlin.cronin@chromium.org</owner>
   <summary>
     Whether or not the uninstallation of an external extension succeeded.
@@ -29033,7 +29040,7 @@ uploading your change for review.
   </summary>
 </histogram>
 
-<histogram name="Extensions.FileAccessAllowed">
+<histogram name="Extensions.FileAccessAllowed" expires_after="2018-08-30">
   <owner>rdevlin.cronin@chromium.org</owner>
   <summary>
     The number of extensions (and friends) that could have been given access to
@@ -29045,7 +29052,7 @@ uploading your change for review.
   </summary>
 </histogram>
 
-<histogram name="Extensions.FileAccessNotAllowed">
+<histogram name="Extensions.FileAccessNotAllowed" expires_after="2018-08-30">
   <owner>rdevlin.cronin@chromium.org</owner>
   <summary>
     The number of extensions (and friends) that could have been given access to
@@ -29362,7 +29369,7 @@ uploading your change for review.
 </histogram>
 
 <histogram name="Extensions.HostedAppUnlimitedStoragePersistentStorageUsage"
-    units="KB">
+    units="KB" expires_after="2018-08-30">
   <owner>rdevlin.cronin@chromium.org</owner>
   <summary>
     The usage (in kilobytes) of persistent storage in a hosted app with the
@@ -29374,7 +29381,7 @@ uploading your change for review.
 </histogram>
 
 <histogram name="Extensions.HostedAppUnlimitedStorageTemporaryStorageUsage"
-    units="%">
+    units="%" expires_after="2018-08-30">
   <owner>rdevlin.cronin@chromium.org</owner>
   <summary>
     The percentage of &quot;normal&quot; (that is, not unlimited) quota that a
@@ -29386,7 +29393,8 @@ uploading your change for review.
   </summary>
 </histogram>
 
-<histogram name="Extensions.HostedAppUnlimitedStorageUsage" units="KB">
+<histogram name="Extensions.HostedAppUnlimitedStorageUsage" units="KB"
+    expires_after="2018-08-30">
   <owner>rdevlin.cronin@chromium.org</owner>
   <summary>
     The usage (in kilobytes) of a hosted app with the unlimitedStorage api
@@ -29408,7 +29416,7 @@ uploading your change for review.
   </summary>
 </histogram>
 
-<histogram name="Extensions.IncognitoNotAllowed">
+<histogram name="Extensions.IncognitoNotAllowed" expires_after="2018-08-30">
   <owner>rdevlin.cronin@chromium.org</owner>
   <summary>
     The number of extensions (and friends) that could have been allowed in
@@ -29460,14 +29468,15 @@ uploading your change for review.
   </summary>
 </histogram>
 
-<histogram name="Extensions.InjectEnd_BlockingScriptCount">
+<histogram name="Extensions.InjectEnd_BlockingScriptCount"
+    expires_after="2019-12-31">
   <owner>rdevlin.cronin@chromium.org</owner>
   <summary>
     Number of blocking scripts injected at document end by extensions.
   </summary>
 </histogram>
 
-<histogram name="Extensions.InjectEnd_ScriptCount">
+<histogram name="Extensions.InjectEnd_ScriptCount" expires_after="2019-12-31">
   <owner>rdevlin.cronin@chromium.org</owner>
   <summary>Number of scripts injected at document end by extensions.</summary>
 </histogram>
@@ -29480,14 +29489,15 @@ uploading your change for review.
   </summary>
 </histogram>
 
-<histogram name="Extensions.InjectIdle_BlockingScriptCount">
+<histogram name="Extensions.InjectIdle_BlockingScriptCount"
+    expires_after="2019-12-31">
   <owner>rdevlin.cronin@chromium.org</owner>
   <summary>
     Number of blocking scripts injected at document idle by extensions.
   </summary>
 </histogram>
 
-<histogram name="Extensions.InjectIdle_ScriptCount">
+<histogram name="Extensions.InjectIdle_ScriptCount" expires_after="2019-12-31">
   <owner>rdevlin.cronin@chromium.org</owner>
   <summary>Number of scripts injected at document idle by extensions.</summary>
 </histogram>
@@ -29515,7 +29525,7 @@ uploading your change for review.
   </summary>
 </histogram>
 
-<histogram name="Extensions.InjectStart_CssCount">
+<histogram name="Extensions.InjectStart_CssCount" expires_after="2019-12-31">
   <owner>rdevlin.cronin@chromium.org</owner>
   <summary>Number of css files injected by extensions.</summary>
 </histogram>
@@ -29525,7 +29535,8 @@ uploading your change for review.
   <summary>Number of scripts injected at document start by extensions.</summary>
 </histogram>
 
-<histogram name="Extensions.InjectStart_Time" units="ms">
+<histogram name="Extensions.InjectStart_Time" units="ms"
+    expires_after="2019-12-31">
   <owner>rdevlin.cronin@chromium.org</owner>
   <summary>
     Time taken to inject css/scripts at document start by extensions. Not
@@ -29582,7 +29593,8 @@ uploading your change for review.
   <summary>The number of extensions and themes loaded at profile open.</summary>
 </histogram>
 
-<histogram name="Extensions.LoadAllComponentTime" units="ms">
+<histogram name="Extensions.LoadAllComponentTime" units="ms"
+    expires_after="2019-12-31">
   <owner>rdevlin.cronin@chromium.org</owner>
   <summary>
     Time taken to load all component extensions at profile open. This happens as
@@ -29591,7 +29603,7 @@ uploading your change for review.
   </summary>
 </histogram>
 
-<histogram name="Extensions.LoadAllTime" units="ms">
+<histogram name="Extensions.LoadAllTime" units="ms" expires_after="2019-12-31">
   <owner>rdevlin.cronin@chromium.org</owner>
   <summary>
     Time taken to load all non-component extensions at profile open. This
@@ -29600,7 +29612,7 @@ uploading your change for review.
   </summary>
 </histogram>
 
-<histogram name="Extensions.LoadAllTime2" units="ms">
+<histogram name="Extensions.LoadAllTime2" units="ms" expires_after="2019-12-31">
   <owner>rdevlin.cronin@chromium.org</owner>
   <summary>
     Time taken to load all non-component extensions at profile open and record
@@ -29621,14 +29633,14 @@ uploading your change for review.
   </summary>
 </histogram>
 
-<histogram name="Extensions.LoadAppUser">
+<histogram name="Extensions.LoadAppUser" expires_after="2019-12-31">
   <owner>rdevlin.cronin@chromium.org</owner>
   <summary>
     The number of user-installed apps loaded by each user at profile open.
   </summary>
 </histogram>
 
-<histogram name="Extensions.LoadBrowserAction">
+<histogram name="Extensions.LoadBrowserAction" expires_after="2019-12-31">
   <owner>rdevlin.cronin@chromium.org</owner>
   <summary>
     The number of browser action extensions loaded at profile open.
@@ -29686,7 +29698,7 @@ uploading your change for review.
   </summary>
 </histogram>
 
-<histogram name="Extensions.LoadNoExtensionAction">
+<histogram name="Extensions.LoadNoExtensionAction" expires_after="2019-12-31">
   <owner>rdevlin.cronin@chromium.org</owner>
   <summary>
     The number of extensions that had neither a page nor browser action
@@ -30378,7 +30390,8 @@ uploading your change for review.
   </summary>
 </histogram>
 
-<histogram name="Extensions.PopupCreateTime" units="ms">
+<histogram name="Extensions.PopupCreateTime" units="ms"
+    expires_after="2019-12-31">
   <owner>rdevlin.cronin@chromium.org</owner>
   <summary>
     The time taken between a user clicking on a button to open a popup, and the
@@ -30398,7 +30411,8 @@ uploading your change for review.
   <summary>The time for an Extension's popup to load.</summary>
 </histogram>
 
-<histogram name="Extensions.PopupLoadTime2" units="ms">
+<histogram name="Extensions.PopupLoadTime2" units="ms"
+    expires_after="2019-12-31">
   <owner>rdevlin.cronin@chromium.org</owner>
   <summary>
     The time taken for an extension popup to load its initial URL. This may not
@@ -30684,7 +30698,8 @@ uploading your change for review.
   </summary>
 </histogram>
 
-<histogram name="Extensions.SyncGetMessageBundle" units="ms">
+<histogram name="Extensions.SyncGetMessageBundle" units="ms"
+    expires_after="2019-12-31">
   <owner>rdevlin.cronin@chromium.org</owner>
   <summary>
     The amount of synchronous time spent retrieving an extension's message
@@ -30701,7 +30716,8 @@ uploading your change for review.
   </summary>
 </histogram>
 
-<histogram name="Extensions.ThrottledNetworkRequests" units="requests">
+<histogram name="Extensions.ThrottledNetworkRequests" units="requests"
+    expires_after="2018-08-30">
   <owner>rdevlin.cronin@chromium.org</owner>
   <summary>
     The total number of network requests that were delayed by extension user
@@ -80325,7 +80341,7 @@ uploading your change for review.
 </histogram>
 
 <histogram name="ProxyOverriddenBubble.UserSelection"
-    units="ExtensionBubbleAction">
+    units="ExtensionBubbleAction" expires_after="2019-12-31">
   <owner>rdevlin.cronin@chromium.org</owner>
   <summary>
     The action taken by the user when the &quot;proxy overridden&quot; extension
@@ -105867,7 +105883,8 @@ uploading your change for review.
   <summary>Number of retries until the final response was recorded.</summary>
 </histogram>
 
-<histogram name="Toolbar.ActionsModel.ComponentActionsCount">
+<histogram name="Toolbar.ActionsModel.ComponentActionsCount"
+    expires_after="2018-08-30">
   <owner>rdevlin.cronin@chromium.org</owner>
   <summary>
     The number of component action icons the Browser Actions Container knows
@@ -105877,7 +105894,8 @@ uploading your change for review.
   </summary>
 </histogram>
 
-<histogram name="Toolbar.ActionsModel.OverallActionsCount">
+<histogram name="Toolbar.ActionsModel.OverallActionsCount"
+    expires_after="2019-12-31">
   <owner>rdevlin.cronin@chromium.org</owner>
   <summary>
     The total number of action icons the Browser Actions Container knows about