[Blink] Extend histogram expiry.

Bug: 1140015
Change-Id: Ife0330b6c187d5693fdc24c44b7930416d35eb5e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2502370Reviewed-by: Mark Pearson <mpearson@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Mark Pearson <mpearson@chromium.org>
Auto-Submit: Mugdha Lakhani <nator@chromium.org>
Cr-Commit-Position: refs/heads/master@{#821521}

tools/metrics/histograms/histograms_xml/blink/histograms.xml

@@ -1816,11 +1816,12 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
 </histogram>
 
 <histogram name="Blink.ThreadedIconLoader.LoadTime" units="ms"
-    expires_after="2020-12-01">
+    expires_after="2021-11-30">
   <owner>nator@chromium.org</owner>
   <owner>rayankans@chromium.org</owner>
   <summary>
     The number of milliseconds it took to finish successfully loading an icon
+    using ThreadedIconLoader. Recorded whenever a blink module loads an icon
     using ThreadedIconLoader.
   </summary>
 </histogram>