Introduce {next,previous}PositionOf() as replacement of {next,previous}() member functions in Positi

Source/core/editing/commands/BreakBlockquoteCommand.cpp

Index: Source/core/editing/commands/BreakBlockquoteCommand.cpp
diff --git a/Source/core/editing/commands/BreakBlockquoteCommand.cpp b/Source/core/editing/commands/BreakBlockquoteCommand.cpp
index 3edaaf05911ec9445d254fcf75b59d622126bcea..c372695d88eb422bb461dd3a84fe0f1a3b0091fe 100644
--- a/Source/core/editing/commands/BreakBlockquoteCommand.cpp
+++ b/Source/core/editing/commands/BreakBlockquoteCommand.cpp
@@ -126,12 +126,18 @@ void BreakBlockquoteCommand::doApply()
 
     // Don't move a line break just after the caret.  Doing so would create an extra, empty paragraph
     // in the new blockquote.
-    if (lineBreakExistsAtVisiblePosition(visiblePos))
-        pos = pos.next();
+    if (lineBreakExistsAtVisiblePosition(visiblePos)) {
+        // TODO(yosin) We should use |PositionMoveType::Character| for
+        // |nextPositionOf()| to avoid editing middle of character.
+        pos = nextPositionOf(pos, PositionMoveType::CodePoint);
+    }
 
     // Adjust the position so we don't split at the beginning of a quote.
-    while (isFirstVisiblePositionInNode(VisiblePosition(pos), toHTMLQuoteElement(enclosingNodeOfType(pos, isMailHTMLBlockquoteElement))))
-        pos = pos.previous();
+    while (isFirstVisiblePositionInNode(VisiblePosition(pos), toHTMLQuoteElement(enclosingNodeOfType(pos, isMailHTMLBlockquoteElement)))) {
+        // TODO(yosin) We should use |PositionMoveType::Character| for
+        // |previousPositionOf()| to avoid editing middle character.
+        pos = previousPositionOf(pos, PositionMoveType::CodePoint);
+    }
 
     // startNode is the first node that we need to move to the new blockquote.
     Node* startNode = pos.anchorNode();