[Sync] Fixing easy lint violations.

components/sync/device_info/device_info_tracker.h

 // Copyright 2014 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_DEVICE_INFO_DEVICE_INFO_TRACKER_H_
 #define COMPONENTS_SYNC_DEVICE_INFO_DEVICE_INFO_TRACKER_H_
 
 #include <memory>
 #include <string>
+#include <vector>
 
 #include "components/sync/device_info/device_info.h"
 
 namespace syncer {
 
 // Interface for tracking synced DeviceInfo.
 class DeviceInfoTracker {
  public:
   virtual ~DeviceInfoTracker() {}
 
@@ skipping 16 matching lines @@
   virtual void AddObserver(Observer* observer) = 0;
   // Unregisters an observer.
   virtual void RemoveObserver(Observer* observer) = 0;
   // Returns the count of active devices.
   virtual int CountActiveDevices() const = 0;
 };
 
 }  // namespace syncer
 
 #endif  // COMPONENTS_SYNC_DEVICE_INFO_DEVICE_INFO_TRACKER_H_