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

Source/core/html/track/TextTrackContainer.cpp

 /*
  * Copyright (C) 2008, 2009, 2010, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2012 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  *
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
@@ skipping 73 matching lines @@
     // everything is handled through CSS. The caption box is on top of the
     // controls box, in a container set with the -webkit-box display property.
 
     // 6. Let tracks be the subset of video's list of text tracks that have as
     // their rules for updating the text track rendering these rules for
     // updating the display of WebVTT text tracks, and whose text track mode is
     // showing or showing by default.
     // 7. Let cues be an empty list of text track cues.
     // 8. For each track track in tracks, append to cues all the cues from
     // track's list of cues that have their text track cue active flag set.
-    CueList activeCues = video.cueTimeline().currentlyActiveCues();
+    const CueList& activeCues = video.cueTimeline().currentlyActiveCues();
 
     // 9. If reset is false, then, for each text track cue cue in cues: if cue's
     // text track cue display state has a set of CSS boxes, then add those boxes
     // to output, and remove cue from cues.
 
     // There is nothing explicitly to be done here, as all the caching occurs
     // within the TextTrackCue instance itself. If parameters of the cue change,
     // the display tree is cleared.
 
     // 10. For each text track cue cue in cues that has not yet had
@@ skipping 10 matching lines @@
     }
 
     // 11. Return output.
     if (hasChildren())
         removeInlineStyleProperty(CSSPropertyDisplay);
     else
         setInlineStyleProperty(CSSPropertyDisplay, CSSValueNone);
 }
 
 } // namespace blink