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: Ie3dea1da3f7a42b0454a79ea1ca77a153629ddc9
Reviewed-on: https://chromium-review.googlesource.com/1126174Reviewed-by: Mark Pearson <mpearson@chromium.org>
Commit-Queue: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#573543}

tools/metrics/histograms/histograms.xml

@@ -33534,7 +33534,8 @@ uploading your change for review.
   </summary>
 </histogram>
 
-<histogram name="History.InMemoryURLIndexingTime" units="ms">
+<histogram name="History.InMemoryURLIndexingTime" units="ms"
+    expires_after="2018-08-30">
   <owner>mpearson@chromium.org</owner>
   <summary>
     Time to rebuild in-memory url index from main history database. Recorded
@@ -33550,7 +33551,8 @@ uploading your change for review.
   </summary>
 </histogram>
 
-<histogram name="History.InMemoryURLIndexSaveCacheTime" units="ms">
+<histogram name="History.InMemoryURLIndexSaveCacheTime" units="ms"
+    expires_after="2018-08-30">
   <owner>mpearson@chromium.org</owner>
   <summary>
     Time to write in-memory url index to disk cache. Recorded once shortly after
@@ -64380,7 +64382,7 @@ uploading your change for review.
   </summary>
 </histogram>
 
-<histogram name="Omnibox.PasteAndGo" units="count">
+<histogram name="Omnibox.PasteAndGo" units="count" expires_after="2018-08-30">
   <owner>mpearson@chromium.org</owner>
   <summary>
     The number of paste-and-go commands on the text in the omnibox. Reported
@@ -64552,7 +64554,8 @@ uploading your change for review.
   </summary>
 </histogram>
 
-<histogram name="Omnibox.SearchProvider.ConvertResultsTime" units="ms">
+<histogram name="Omnibox.SearchProvider.ConvertResultsTime" units="ms"
+    expires_after="2018-08-30">
   <owner>mpearson@chromium.org</owner>
   <summary>
     Time it takes to convert all the results to matches and add them to a map,
@@ -64562,7 +64565,7 @@ uploading your change for review.
 
 <histogram
     name="Omnibox.SearchProvider.GetMostRecentKeywordTermsDefaultProviderTime"
-    units="ms">
+    units="ms" expires_after="2018-08-30">
   <owner>mpearson@chromium.org</owner>
   <summary>
     Time it takes for the omnibox to search the previous query history database
@@ -91123,7 +91126,8 @@ uploading your change for review.
   </summary>
 </histogram>
 
-<histogram name="Settings.PinnedTabEngineTypes" enum="OmniboxSearchEngineType">
+<histogram name="Settings.PinnedTabEngineTypes" enum="OmniboxSearchEngineType"
+    expires_after="2018-08-30">
   <owner>mpearson@chromium.org</owner>
   <summary>
     Tries to pretend pinned tab URLs are search URLs, and records the search
@@ -91209,7 +91213,8 @@ uploading your change for review.
   </summary>
 </histogram>
 
-<histogram name="Settings.ShowHomeButton.PulledFromSync" enum="BooleanEnabled">
+<histogram name="Settings.ShowHomeButton.PulledFromSync" enum="BooleanEnabled"
+    expires_after="2018-08-30">
   <owner>mpearson@chromium.org</owner>
   <summary>
     The enabled state of the Home button pref when pulled down from sync to
@@ -91217,7 +91222,8 @@ uploading your change for review.
   </summary>
 </histogram>
 
-<histogram name="Settings.ShowHomeButton.PushedToSync" enum="BooleanEnabled">
+<histogram name="Settings.ShowHomeButton.PushedToSync" enum="BooleanEnabled"
+    expires_after="2018-08-30">
   <owner>mpearson@chromium.org</owner>
   <summary>
     The enabled state of the Home button pref when pushed up to sync from a
@@ -91263,7 +91269,7 @@ uploading your change for review.
 </histogram>
 
 <histogram name="Settings.StartupPageEngineTypes"
-    enum="OmniboxSearchEngineType">
+    enum="OmniboxSearchEngineType" expires_after="2018-08-30">
   <owner>mpearson@chromium.org</owner>
   <summary>
     Tries to pretend the startup page URLs are search URLs, and records the
@@ -91280,7 +91286,7 @@ uploading your change for review.
 </histogram>
 
 <histogram name="Settings.StartupPageLoadSettings.PulledFromSync"
-    enum="SessionStartupPref">
+    enum="SessionStartupPref" expires_after="2018-08-30">
   <owner>mpearson@chromium.org</owner>
   <summary>
     The startup page setting when pulled down from sync to update an out-of-sync
@@ -91289,7 +91295,7 @@ uploading your change for review.
 </histogram>
 
 <histogram name="Settings.StartupPageLoadSettings.PushedToSync"
-    enum="SessionStartupPref">
+    enum="SessionStartupPref" expires_after="2018-08-30">
   <owner>mpearson@chromium.org</owner>
   <summary>
     The startup page setting when pushed up to sync from a change made locally.
@@ -91902,7 +91908,7 @@ uploading your change for review.
   </summary>
 </histogram>
 
-<histogram name="ShortcutsProvider.DatabaseSize">
+<histogram name="ShortcutsProvider.DatabaseSize" expires_after="2018-08-30">
   <owner>mpearson@chromium.org</owner>
   <summary>
     The number of entries in shortcuts backend's database when initialized,