[Line Layout API] Add textStartOffset to LineLayoutText

Source/core/layout/api/LineLayoutText.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef LineLayoutText_h
 #define LineLayoutText_h
 
 #include "core/layout/LayoutText.h"
 #include "core/layout/api/LineLayoutItem.h"
 #include "platform/LayoutUnit.h"
@@ skipping 104 matching lines @@
     SelectionState selectionState() const
     {
         return toText()->selectionState();
     }
 
     void selectionStartEnd(int& spos, int& epos) const
     {
         return toText()->selectionStartEnd(spos, epos);
     }
 
+    unsigned textStartOffset() const
+    {
+        return toText()->textStartOffset();
+    }
+
 private:
     LayoutText* toText() { return toLayoutText(layoutObject()); }
     const LayoutText* toText() const { return toLayoutText(layoutObject()); }
 };
 
 } // namespace blink
 
 #endif // LineLayoutText_h