Introduce composed tree version of startOfWord()

Source/core/editing/VisibleUnits.h

Index: Source/core/editing/VisibleUnits.h
diff --git a/Source/core/editing/VisibleUnits.h b/Source/core/editing/VisibleUnits.h
index 23c3b687a00b779c8361c9115b6c09bb5539434e..acfd230c8d39c7fd59bb4700eddfaf4693c7922c 100644
--- a/Source/core/editing/VisibleUnits.h
+++ b/Source/core/editing/VisibleUnits.h
@@ -115,6 +115,7 @@ CORE_EXPORT VisiblePositionInComposedTree previousPositionOf(const VisiblePositi
 
 // words
 CORE_EXPORT VisiblePosition startOfWord(const VisiblePosition&, EWordSide = RightWordIfOnBoundary);
+CORE_EXPORT VisiblePositionInComposedTree startOfWord(const VisiblePositionInComposedTree&, EWordSide = RightWordIfOnBoundary);
 CORE_EXPORT VisiblePosition endOfWord(const VisiblePosition&, EWordSide = RightWordIfOnBoundary);
 VisiblePosition previousWordPosition(const VisiblePosition&);
 VisiblePosition nextWordPosition(const VisiblePosition&);