Let CueTimeline::currentlyActiveCues() return by reference-to-const

Source/core/html/track/CueTimeline.h

 // Copyright 2015 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 CueTimeline_h
 #define CueTimeline_h
 
 #include "core/html/track/TextTrackCue.h"
 #include "core/html/track/vtt/VTTCue.h"
 #include "platform/PODIntervalTree.h"
@@ skipping 21 matching lines @@
     void removeCue(TextTrack*, PassRefPtrWillBeRawPtr<TextTrackCue>);
 
     void hideCues(TextTrack*, const TextTrackCueList*);
 
     void updateActiveCues(double);
 
     bool ignoreUpdateRequests() const { return m_ignoreUpdate > 0; }
     void beginIgnoringUpdateRequests();
     void endIgnoringUpdateRequests();
 
-    CueList currentlyActiveCues() const { return m_currentlyActiveCues; }
+    const CueList& currentlyActiveCues() const { return m_currentlyActiveCues; }
 
     DECLARE_TRACE();
 
 private:
     HTMLMediaElement& mediaElement() const { return *m_mediaElement; }
 
     void addCueInternal(PassRefPtrWillBeRawPtr<TextTrackCue>);
     void removeCueInternal(PassRefPtrWillBeRawPtr<TextTrackCue>);
 
     RawPtrWillBeMember<HTMLMediaElement> m_mediaElement;
@@ skipping 39 matching lines @@
     static String string(TextTrackCue* const& cue)
     {
         return cue->toString();
     }
 };
 #endif
 
 } // namespace blink
 
 #endif // CueTimeline_h