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

chrome/browser/sync/sessions/session_state.h

Index: chrome/browser/sync/sessions/session_state.h
diff --git a/chrome/browser/sync/sessions/session_state.h b/chrome/browser/sync/sessions/session_state.h
index c0c5f39ada3f75936e00d36a5c9dafeab4de05b2..41ec77e096426812a43383f3f5abb1e4b45d5844 100644
--- a/chrome/browser/sync/sessions/session_state.h
+++ b/chrome/browser/sync/sessions/session_state.h
@@ -127,7 +127,8 @@ struct SyncSessionSnapshot {
       int num_blocking_conflicting_updates,
       int num_conflicting_updates,
       bool did_commit_items,
-      const SyncSourceInfo& source);
+      const SyncSourceInfo& source,
+      size_t num_entries);
   ~SyncSessionSnapshot();
 
   // Caller takes ownership of the returned dictionary.
@@ -146,6 +147,7 @@ struct SyncSessionSnapshot {
   const int num_conflicting_updates;
   const bool did_commit_items;
   const SyncSourceInfo source;
+  const size_t num_entries;
 };
 
 // Tracks progress of conflicts and their resolution using conflict sets.