[Sync] Replacing NULL with nullptr/null throughout sync code.

components/sync_sessions/synced_session_tracker.h

Index: components/sync_sessions/synced_session_tracker.h
diff --git a/components/sync_sessions/synced_session_tracker.h b/components/sync_sessions/synced_session_tracker.h
index 63e8429fd0bf7c6f7c55d8b2524246082a8b29e5..35123d76a9ce54e67999f31071da0feac5128723 100644
--- a/components/sync_sessions/synced_session_tracker.h
+++ b/components/sync_sessions/synced_session_tracker.h
@@ -67,7 +67,7 @@ class SyncedSessionTracker {
   // If lookup succeeds:
   // - Sets tab to point to the SessionTab, and returns true.
   // Else
-  // - Returns false, tab is set to NULL.
+  // - Returns false, tab is set to null.
   bool LookupSessionTab(const std::string& session_tag,
                         SessionID::id_type tab_id,
                         const sessions::SessionTab** tab) const;