Deprecate old GC idle time histograms.

The patch marks the following histograms as obsolete:
- V8.GCIdleTimeAllottedInMS
- V8.GCIdleTimeLimit.Overshot
- V8.GCIdleTimeLimit.Undershot

BUG=chromium:751045

Change-Id: I78205e419373f38cc76d45599163cb0d87eabf68
Reviewed-on: https://chromium-review.googlesource.com/594729
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491829}

tools/metrics/histograms/histograms.xml

@@ -83873,11 +83873,17 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
 </histogram>
 
 <histogram name="V8.GCIdleTimeAllottedInMS" units="ms">
+  <obsolete>
+    Deprecated in August 2017. No longer tracked.
+  </obsolete>
   <owner>hpayer@chromium.org</owner>
   <summary>Idle time passed to V8 via IdleNotifications.</summary>
 </histogram>
 
 <histogram name="V8.GCIdleTimeLimit.Overshot" units="ms">
+  <obsolete>
+    Deprecated in August 2017. No longer tracked.
+  </obsolete>
   <owner>hpayer@chromium.org</owner>
   <summary>
     Milliseconds the idle time limit was overshot by the IdleNotification.
@@ -83885,6 +83891,9 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
 </histogram>
 
 <histogram name="V8.GCIdleTimeLimit.Undershot" units="ms">
+  <obsolete>
+    Deprecated in August 2017. No longer tracked.
+  </obsolete>
   <owner>hpayer@chromium.org</owner>
   <summary>
     Milliseconds the idle time limit was undershot by the IdleNotification.