Make ApplyBlockElementCommand not to modify uneditable element

Source/core/editing/ApplyBlockElementCommand.cpp

Index: Source/core/editing/ApplyBlockElementCommand.cpp
diff --git a/Source/core/editing/ApplyBlockElementCommand.cpp b/Source/core/editing/ApplyBlockElementCommand.cpp
index fbffc837bdd908919ad94ad791b904795bb6c336..c23350cb5e6e58f85cb4d54eefaba9dedeed69b5 100644
--- a/Source/core/editing/ApplyBlockElementCommand.cpp
+++ b/Source/core/editing/ApplyBlockElementCommand.cpp
@@ -227,7 +227,7 @@ void ApplyBlockElementCommand::rangeForParagraphSplittingTextNodesIfNeeded(const
         }
 
         // If end is in the middle of a text node, split.
-        if (!endStyle->collapseWhiteSpace() && end.offsetInContainerNode() && end.offsetInContainerNode() < end.computeContainerNode()->maxCharacterOffset()) {
+        if (endStyle->userModify() != READ_ONLY && !endStyle->collapseWhiteSpace() && end.offsetInContainerNode() && end.offsetInContainerNode() < end.computeContainerNode()->maxCharacterOffset()) {
             RefPtrWillBeRawPtr<Text> endContainer = toText(end.computeContainerNode());
             splitTextNode(endContainer, end.offsetInContainerNode());
             if (isStartAndEndOnSameNode)