Have positionInParentBeforeNode() / positionInParentAfterNode() take a reference

Source/core/editing/InsertTextCommand.cpp

Index: Source/core/editing/InsertTextCommand.cpp
diff --git a/Source/core/editing/InsertTextCommand.cpp b/Source/core/editing/InsertTextCommand.cpp
index 403da8a46fbb6ef9c1d5404dbd7de9152e6bc148..da200d7c27f24d5cda2599c8d61f4ad3b18f9982 100644
--- a/Source/core/editing/InsertTextCommand.cpp
+++ b/Source/core/editing/InsertTextCommand.cpp
@@ -164,7 +164,8 @@ void InsertTextCommand::doApply()
 
     // It is possible for the node that contains startPosition to contain only unrendered whitespace,
     // and so deleteInsignificantText could remove it.  Save the position before the node in case that happens.
-    Position positionBeforeStartNode(positionInParentBeforeNode(startPosition.containerNode()));
+    ASSERT(startPosition.containerNode());
+    Position positionBeforeStartNode(positionInParentBeforeNode(*startPosition.containerNode()));
     deleteInsignificantText(startPosition, startPosition.downstream());
     if (!startPosition.inDocument())
         startPosition = positionBeforeStartNode;