[subresource_filter] Remove SubresourceFilter.PageLoad.BlockedPopups

This metric is broken, and we are moving to a new system for blocking
popups anyway.

The new metric will be landed here:
https://chromium-review.googlesource.com/c/chromium/src/+/726545

Bug: 772946
Change-Id: Ib6ce9d316f7188be13a53988e06623ee93065df3
Reviewed-on: https://chromium-review.googlesource.com/728953
Reviewed-by: Shivani Sharma <shivanisha@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Commit-Queue: Charlie Harrison <csharrison@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#510525}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 655b3cccf128cb8ee6f000a871ff0726b6134ca9
diff --git a/metrics/histograms/histograms.xml b/metrics/histograms/histograms.xml
index 2b28963..8741cec 100644
--- a/metrics/histograms/histograms.xml
+++ b/metrics/histograms/histograms.xml
@@ -81926,6 +81926,12 @@
 </histogram>
 
 <histogram name="SubresourceFilter.PageLoad.BlockedPopups" units="popups">
+  <obsolete>
+    Metric is broken and will be replaced by
+    ContentSettings.Popups.StringBlocked.NumBlocked. This only logged values for
+    sites which are also activated for subresource filtering, when most sites
+    will probably only be marked for popup blocking.
+  </obsolete>
   <owner>csharrison@chromium.org</owner>
   <summary>
     Records the number of popups blocked per page load due to subresource filter