Add information about sync session and other useful data to about:sync.

chrome/browser/sync/engine/all_status.cc

Index: chrome/browser/sync/engine/all_status.cc
diff --git a/chrome/browser/sync/engine/all_status.cc b/chrome/browser/sync/engine/all_status.cc
index 83d148abf0d04c88875f73b4b76eb30fe9c4231d..47f0e2a674046952d3dbcd362954ae7a73dfd840 100644
--- a/chrome/browser/sync/engine/all_status.cc
+++ b/chrome/browser/sync/engine/all_status.cc
@@ -79,6 +79,12 @@ sync_api::SyncManager::Status AllStatus::CalcSyncing(
         snapshot->syncer_status.num_local_overwrites;
     status.num_server_overwrites_total +=
         snapshot->syncer_status.num_server_overwrites;
+    if (snapshot->syncer_status.num_successful_commits == 0 &&
+        snapshot->syncer_status.num_updates_downloaded_total == 0) {
+      ++status.useless_sync_cycles;
+    } else {
+      ++status.useful_sync_cycles;
+    }
   }
   return status;
 }