Make previousPositionOf() to handle yet another no previous position case

third_party/WebKit/Source/core/editing/VisibleUnits.cpp

 /*
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009 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 3214 matching lines @@
 
     // return null visible position if there is no previous visible position
     if (pos.atStartOfTree())
         return VisiblePositionTemplate<Strategy>();
 
     // we should always be able to make the affinity |TextAffinity::Downstream|,
     // because going previous from an |TextAffinity::Upstream| position can
     // never yield another |TextAffinity::Upstream position| (unless line wrap
     // length is 0!).
     const VisiblePositionTemplate<Strategy> prev = createVisiblePosition(pos);
-    DCHECK_NE(prev.deepEquivalent(), visiblePosition.deepEquivalent());
+    if (prev.deepEquivalent() == visiblePosition.deepEquivalent())
+        return VisiblePositionTemplate<Strategy>();
 
     switch (rule) {
     case CanCrossEditingBoundary:
         return prev;
     case CannotCrossEditingBoundary:
         return honorEditingBoundaryAtOrBefore(prev, visiblePosition.deepEquivalent());
     case CanSkipOverEditingBoundary:
         return skipToStartOfEditingBoundary(prev, visiblePosition.deepEquivalent());
     }
 
     NOTREACHED();
     return honorEditingBoundaryAtOrBefore(prev, visiblePosition.deepEquivalent());
 }
 
 VisiblePosition previousPositionOf(const VisiblePosition& visiblePosition, EditingBoundaryCrossingRule rule)
 {
     return previousPositionOfAlgorithm<EditingStrategy>(visiblePosition, rule);
 }
 
 VisiblePositionInFlatTree previousPositionOf(const VisiblePositionInFlatTree& visiblePosition, EditingBoundaryCrossingRule rule)
 {
     return previousPositionOfAlgorithm<EditingInFlatTreeStrategy>(visiblePosition, rule);
 }
 
 } // namespace blink