Obsolete AnnouncementNotificationEvent in enums.xml.

The code that uses this enum is deleted in:
https://chromium-review.googlesource.com/c/chromium/src/+/2486031

This CL marks the enum deprecated in enums.xml.

TBR=bcwhite@chromium.org

Bug: 1130635
Change-Id: Ib148fc0e6632aa5fc70db2c8c5d82e88a59178c0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2551297Reviewed-by: Xing Liu <xingliu@chromium.org>
Commit-Queue: Xing Liu <xingliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#829923}

tools/metrics/histograms/enums.xml

@@ -2403,6 +2403,9 @@ Unknown properties are collapsed to zero. -->
 </enum>
 
 <enum name="AnnouncementNotificationEvent">
+  <obsolete>
+    Deprecated 11/2020
+  </obsolete>
   <int value="0" label="Start"/>
   <int value="1" label="Show notification"/>
   <int value="2" label="Click"/>