Introduce isRenderedCharacter(Position) function

Source/core/editing/commands/InsertLineBreakCommand.cpp

Index: Source/core/editing/commands/InsertLineBreakCommand.cpp
diff --git a/Source/core/editing/commands/InsertLineBreakCommand.cpp b/Source/core/editing/commands/InsertLineBreakCommand.cpp
index da3d3a21df0d7420f2b2035fdcf5d856122e211c..4539d2996449ce9a809c698429a543ce1665f563 100644
--- a/Source/core/editing/commands/InsertLineBreakCommand.cpp
+++ b/Source/core/editing/commands/InsertLineBreakCommand.cpp
@@ -123,7 +123,9 @@ void InsertLineBreakCommand::doApply()
 
         // Handle whitespace that occurs after the split
         document().updateLayoutIgnorePendingStylesheets();
-        if (!endingPosition.isRenderedCharacter()) {
+        // TODO(yosin) |isRenderedCharacter()| should be removed, and we should
+        // use |VisiblePosition::characterAfter()|.
+        if (!isRenderedCharacter(endingPosition)) {
             Position positionBeforeTextNode(positionInParentBeforeNode(*textNode));
             // Clear out all whitespace and insert one non-breaking space
             deleteInsignificantTextDownstream(endingPosition);