Extend/expire Sync histograms

Sync.URLFetchTimedOut is extended by 6 months. I also updated the
description to point out a (small) caveat with the numbers.
Sync.SessionTabs is marked as obsolete.

Bug: 1160835
Change-Id: If5fd2501742197aa0c09fa1794b828705277a03c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2640035Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Caitlin Fischer <caitlinfischer@google.com>
Commit-Queue: Caitlin Fischer <caitlinfischer@google.com>
Auto-Submit: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#845687}

components/sync_sessions/local_session_event_handler_impl.cc

@@ -10,9 +10,6 @@
 #include <vector>
 
 #include "base/bind.h"
-#include "base/metrics/histogram_macros.h"
-#include "base/strings/stringprintf.h"
-#include "build/build_config.h"
 #include "components/sync/model/sync_change.h"
 #include "components/sync/protocol/sync.pb.h"
 #include "components/sync_sessions/switches.h"
@@ -157,11 +154,6 @@ void LocalSessionEventHandlerImpl::AssociateWindows(ReloadTabsOption option,
 
   for (auto& window_iter_pair : windows) {
     const SyncedWindowDelegate* window_delegate = window_iter_pair.second;
-    if (option == RELOAD_TABS) {
-      UMA_HISTOGRAM_COUNTS_1M("Sync.SessionTabs",
-                              window_delegate->GetTabCount());
-    }
-
     // Make sure the window is viewable and is not about to be closed. The
     // viewable window check is necessary because, for example, when a browser
     // is closed the destructor is not necessarily run immediately. This means

tools/metrics/histograms/histograms_xml/sync/histograms.xml

@@ -905,6 +905,9 @@ can be compared for the same population). -->
 </histogram>
 
 <histogram name="Sync.SessionTabs" units="tabs" expires_after="2021-01-31">
+  <obsolete>
+    Removed 2021-01.
+  </obsolete>
   <owner>mastiz@chromium.org</owner>
   <owner>treib@chromium.org</owner>
   <summary>
@@ -1125,12 +1128,14 @@ can be compared for the same population). -->
 </histogram>
 
 <histogram name="Sync.URLFetchTimedOut" enum="BooleanTimedOut"
-    expires_after="2021-01-31">
+    expires_after="2021-07-31">
   <owner>mastiz@chromium.org</owner>
   <owner>treib@chromium.org</owner>
   <summary>
-    Whether a url fetch timed out or not. Timing out implies the fetch was
-    stalled for an unknown reason.
+    Whether a URL fetch timed out or not. Timing out implies the fetch was
+    stalled for an unknown reason. Note that this records true on timeout and
+    false on success, but doesn't record anything if the fetch failed for any
+    other reason.
   </summary>
 </histogram>