Introduce composed tree version of canonicalPosition() in VisiblePosition

Source/core/editing/VisiblePosition.cpp

Index: Source/core/editing/VisiblePosition.cpp
diff --git a/Source/core/editing/VisiblePosition.cpp b/Source/core/editing/VisiblePosition.cpp
index 3f3073bd117da7ffe597cc6df1709663c166e124..e5e6d2ce29e936e80d9a361825f76b1fcca2a0e2 100644
--- a/Source/core/editing/VisiblePosition.cpp
+++ b/Source/core/editing/VisiblePosition.cpp
@@ -637,6 +637,11 @@ Position canonicalPositionOf(const Position& position)
     return canonicalPosition(position);
 }
 
+PositionInComposedTree canonicalPositionOf(const PositionInComposedTree& position)
+{
+    return canonicalPosition(position);
+}
+
 template<typename PositionType>
 void VisiblePosition::init(const PositionType& position, EAffinity affinity)
 {