SVG: Add comment to |loopbreaker| local static var that it is safe

Source/core/svg/animation/SVGSMILElement.cpp

 /*
  * Copyright (C) 2008 Apple 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.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 1229 matching lines @@
             smilEndEventSender().dispatchEventSoon(this);
     }
 
     m_nextProgressTime = calculateNextProgressTime(elapsed);
     return animationIsContributing;
 }
 
 void SVGSMILElement::notifyDependentsIntervalChanged()
 {
     ASSERT(m_intervalBegin.isFinite());
+    // |loopbreaker| is used to avoid infinite recursions which may be caused from:

[haraken, 2014/05/30 01:39:45]

loopBreaker

+    // |notifyDependentsIntervalChanged| -> |createInstanceTimesFromSyncbase| -> |add{Begin,End}Time| -> |{begin,end}TimeChanged| -> |notifyDependentsIntervalChanged|
+    // |loopbreaker| is defined as a Persistent<HeapHashSet<Member<SVGSMILElement> > >. This is safe because it is guaranteed to be empty after the root |notifyDependentsIntervalChanged| has exit.

[haraken, 2014/05/30 01:39:45]

this is safe => this won't cause leaks
has exit => has exited

     DEFINE_STATIC_LOCAL(OwnPtrWillBePersistent<WillBeHeapHashSet<RawPtrWillBeMember<SVGSMILElement> > >, loopBreaker, (adoptPtrWillBeNoop(new WillBeHeapHashSet<RawPtrWillBeMember<SVGSMILElement> >())));
     if (!loopBreaker->add(this).isNewEntry)
         return;
 
     TimeDependentSet::iterator end = m_syncBaseDependents.end();
     for (TimeDependentSet::iterator it = m_syncBaseDependents.begin(); it != end; ++it) {
         SVGSMILElement* dependent = *it;
         dependent->createInstanceTimesFromSyncbase(this);
     }
 
@@ skipping 92 matching lines @@
 void SVGSMILElement::trace(Visitor* visitor)
 {
     visitor->trace(m_targetElement);
     visitor->trace(m_timeContainer);
     visitor->trace(m_conditions);
     visitor->trace(m_syncBaseDependents);
     SVGElement::trace(visitor);
 }
 
 }