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

components/sync_sessions/synced_session_tracker.h

 // Copyright 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef COMPONENTS_SYNC_SESSIONS_SYNCED_SESSION_TRACKER_H_
 #define COMPONENTS_SYNC_SESSIONS_SYNCED_SESSION_TRACKER_H_
 
 #include <stddef.h>
 
 #include <map>
@@ skipping 49 matching lines @@
   // - Returns false.
   bool LookupSessionWindows(
       const std::string& session_tag,
       std::vector<const sessions::SessionWindow*>* windows) const;
 
   // Attempts to look up the tab associated with the given tag and tab id.
   // Ownership of the SessionTab remains within the 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;
 
   // Allows retrieval of existing data for the local session. Unlike GetSession
   // this won't create-if-not-present.
   bool LookupLocalSession(const SyncedSession** output) const;
 
   // Returns a pointer to the SyncedSession object associated with
   // |session_tag|. If none exists, creates one. Ownership of the
@@ skipping 121 matching lines @@
   // The tag for this machine's local session, so we can distinguish the foreign
   // sessions.
   std::string local_session_tag_;
 
   DISALLOW_COPY_AND_ASSIGN(SyncedSessionTracker);
 };
 
 }  // namespace sync_sessions
 
 #endif  // COMPONENTS_SYNC_SESSIONS_SYNCED_SESSION_TRACKER_H_