Introduce previousPositionOf() for VisiblePosition

Source/core/editing/commands/BreakBlockquoteCommand.cpp

 /*
  * Copyright (C) 2005 Apple Computer, 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 32 matching lines @@
 namespace {
 
 bool isFirstVisiblePositionInNode(const VisiblePosition& visiblePosition, const ContainerNode* node)
 {
     if (visiblePosition.isNull())
         return false;
 
     if (!visiblePosition.deepEquivalent().computeContainerNode()->isDescendantOf(node))
         return false;
 
-    VisiblePosition previous = visiblePosition.previous();
+    VisiblePosition previous = previousPositionOf(visiblePosition);
     return previous.isNull() || !previous.deepEquivalent().anchorNode()->isDescendantOf(node);
 }
 
 bool isLastVisiblePositionInNode(const VisiblePosition& visiblePosition, const ContainerNode* node)
 {
     if (visiblePosition.isNull())
         return false;
 
     if (!visiblePosition.deepEquivalent().computeContainerNode()->isDescendantOf(node))
         return false;
@@ skipping 154 matching lines @@
 
     // Make sure the cloned block quote renders.
     addBlockPlaceholderIfNeeded(clonedBlockquote.get());
 
     // Put the selection right before the break.
     setEndingSelection(VisibleSelection(positionBeforeNode(breakElement.get()), TextAffinity::Downstream, endingSelection().isDirectional()));
     rebalanceWhitespace();
 }
 
 } // namespace blink