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: I538205f53ba84bf5b2aaf4104a09c080e9aab375
Reviewed-on: https://chromium-review.googlesource.com/1126162Reviewed-by: Erik Chen <erikchen@chromium.org>
Commit-Queue: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#573500}

tools/metrics/histograms/histograms.xml

@@ -14098,7 +14098,8 @@ uploading your change for review.
   <summary>The amount of time (ms) to migrate a v7 database to v8.</summary>
 </histogram>
 
-<histogram name="Cookie.TimeDatabaseMigrationToV9" units="ms">
+<histogram name="Cookie.TimeDatabaseMigrationToV9" units="ms"
+    expires_after="2018-08-30">
   <owner>erikchen@chromium.org</owner>
   <summary>The amount of time (ms) to migrate a v8 database to v9.</summary>
 </histogram>
@@ -14151,7 +14152,8 @@ uploading your change for review.
   </summary>
 </histogram>
 
-<histogram name="Cookie.TimeLoadDBQueueWait" units="ms">
+<histogram name="Cookie.TimeLoadDBQueueWait" units="ms"
+    expires_after="2018-08-30">
   <owner>erikchen@chromium.org</owner>
   <summary>
     This histogram records the wall-clock delay between the Load task being
@@ -65070,7 +65072,8 @@ uploading your change for review.
   </summary>
 </histogram>
 
-<histogram name="OSX.BluetoothAvailability" enum="BluetoothAvailability">
+<histogram name="OSX.BluetoothAvailability" enum="BluetoothAvailability"
+    expires_after="2018-08-30">
   <owner>erikchen@chromium.org</owner>
   <summary>
     The availability and capabilities of the Bluetooth driver on OSX devices.
@@ -65117,7 +65120,8 @@ uploading your change for review.
   </summary>
 </histogram>
 
-<histogram name="OSX.Fullscreen.Enter.Style" enum="OSXFullscreenStyle">
+<histogram name="OSX.Fullscreen.Enter.Style" enum="OSXFullscreenStyle"
+    expires_after="2018-08-30">
   <owner>erikchen@chromium.org</owner>
   <summary>
     This event is recorded each time a user triggers fullscreen for a browser
@@ -65127,7 +65131,7 @@ uploading your change for review.
 </histogram>
 
 <histogram name="OSX.Fullscreen.Enter.WindowLocation"
-    enum="OSXFullscreenWindowLocation">
+    enum="OSXFullscreenWindowLocation" expires_after="2018-08-30">
   <owner>erikchen@chromium.org</owner>
   <summary>
     This event is recorded each time a user triggers fullscreen for a browser
@@ -65144,7 +65148,8 @@ uploading your change for review.
   </summary>
 </histogram>
 
-<histogram name="OSX.Handoff.Origin" enum="OSXHandoffOrigin">
+<histogram name="OSX.Handoff.Origin" enum="OSXHandoffOrigin"
+    expires_after="2018-08-30">
   <owner>erikchen@chromium.org</owner>
   <summary>
     This event is recorded each time a Handoff is received by Chrome on OSX. The
@@ -65230,7 +65235,8 @@ uploading your change for review.
   </summary>
 </histogram>
 
-<histogram name="OSX.SystemHotkeyMap.LoadSuccess" enum="BooleanSuccess">
+<histogram name="OSX.SystemHotkeyMap.LoadSuccess" enum="BooleanSuccess"
+    expires_after="2018-08-30">
   <owner>erikchen@chromium.org</owner>
   <summary>
     Whether an attempt to load the system hotkeys on a Mac was successful.
@@ -91778,7 +91784,8 @@ uploading your change for review.
   </summary>
 </histogram>
 
-<histogram name="SharedMemory.CreateError" enum="SharedMemoryCreateError">
+<histogram name="SharedMemory.CreateError" enum="SharedMemoryCreateError"
+    expires_after="2018-08-30">
   <owner>erikchen@chromium.org</owner>
   <summary>
     A histogram entry is emitted each time a base::SharedMemory object is
@@ -96377,7 +96384,8 @@ uploading your change for review.
   <summary>The calling application (if any).</summary>
 </histogram>
 
-<histogram name="Startup.OSX.AwakeFromNib" units="ms">
+<histogram name="Startup.OSX.AwakeFromNib" units="ms"
+    expires_after="2018-08-30">
   <owner>erikchen@chromium.org</owner>
   <summary>
     The amount of time that elapsed between main entry and the invocation of
@@ -96410,7 +96418,8 @@ uploading your change for review.
   </summary>
 </histogram>
 
-<histogram name="Startup.OSX.PreMainMessageLoopStart" units="ms">
+<histogram name="Startup.OSX.PreMainMessageLoopStart" units="ms"
+    expires_after="2018-08-30">
   <owner>erikchen@chromium.org</owner>
   <summary>
     The amount of time that elapsed between main entry and the invocation of
@@ -96418,7 +96427,8 @@ uploading your change for review.
   </summary>
 </histogram>
 
-<histogram name="Startup.OSX.PreProfileInit" units="ms">
+<histogram name="Startup.OSX.PreProfileInit" units="ms"
+    expires_after="2018-08-30">
   <owner>erikchen@chromium.org</owner>
   <summary>
     The amount of time that elapsed between main entry and the invocation of
@@ -96426,7 +96436,8 @@ uploading your change for review.
   </summary>
 </histogram>
 
-<histogram name="Startup.OSX.WillFinishLaunching" units="ms">
+<histogram name="Startup.OSX.WillFinishLaunching" units="ms"
+    expires_after="2018-08-30">
   <owner>erikchen@chromium.org</owner>
   <summary>
     The amount of time that elapsed between main entry and the invocation of