Deprecate histogram FCMInvalidations.FCMMessagesDeleted

Bug: 1052983
Change-Id: I9c40475de027c1d7e734d142d5bbbbdaaf5e700f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2061753
Auto-Submit: Marc Treib <treib@chromium.org>
Reviewed-by: Alex Ilin <alexilin@chromium.org>
Commit-Queue: Alex Ilin <alexilin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#742129}

components/invalidation/impl/fcm_network_handler.cc

@@ -311,10 +311,7 @@ void FCMNetworkHandler::OnMessage(const std::string& app_id,
 
 void FCMNetworkHandler::OnMessagesDeleted(const std::string& app_id) {
   DCHECK_EQ(app_id, app_id_);
-  base::UmaHistogramBoolean("FCMInvalidations.FCMMessagesDeleted", true);
-  // Note: If this actually happens in practice, consider notifying the client
-  // that messages were deleted so it can act on it, e.g. in case of sync by
-  // triggering a GetUpdates.
+  // Note: As of 2020-02, this doesn't actually happen in practice.
 }
 
 void FCMNetworkHandler::OnSendError(

tools/metrics/histograms/histograms.xml

@@ -54586,6 +54586,9 @@ uploading your change for review.
 
 <histogram name="FCMInvalidations.FCMMessagesDeleted"
     enum="BooleanDeletedOrNot" expires_after="M82">
+  <obsolete>
+    Removed 2020-02.
+  </obsolete>
   <owner>treib@chromium.org</owner>
   <owner>melandory@chromium.org</owner>
   <summary>