[Extensions Metrics] Mark Extensions.ThrottledNetworkRequests obsolete

Mark Extensions.ThrottledNetworkRequests as obsolete. It has been
expired since 2018-08. We haven't been looking at it lately, and we
can always add it back later if we need it again.

Bug: 878106
Change-Id: I24fc5e7105976da3cbfbfeb52110e8483edc7c8a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1600859Reviewed-by: Karan Bhatia <karandeepb@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#658275}

chrome/browser/extensions/user_script_listener.cc

@@ -142,8 +142,6 @@ bool UserScriptListener::ShouldDelayRequest(const GURL& url) {
 }
 
 void UserScriptListener::StartDelayedRequests() {
-  UMA_HISTOGRAM_COUNTS_100("Extensions.ThrottledNetworkRequests",
-                           throttles_.size());
   WeakThrottleList::const_iterator it;
   for (it = throttles_.begin(); it != throttles_.end(); ++it) {
     if (it->get())

tools/metrics/histograms/histograms.xml

@@ -39004,6 +39004,9 @@ uploading your change for review.
 
 <histogram name="Extensions.ThrottledNetworkRequests" units="requests"
     expires_after="2018-08-30">
+  <obsolete>
+    Expired 2018-08.
+  </obsolete>
   <owner>rdevlin.cronin@chromium.org</owner>
   <summary>
     The total number of network requests that were delayed by extension user