Cleanup LayoutTextControl and its subclasses.

Source/core/layout/LayoutTextControlSingleLine.cpp

Index: Source/core/layout/LayoutTextControlSingleLine.cpp
diff --git a/Source/core/layout/LayoutTextControlSingleLine.cpp b/Source/core/layout/LayoutTextControlSingleLine.cpp
index c6e2a51af97bba24f1c5ce1dde15ac5273381815..bf8ec2b730dad4147596d39d95d85dae3816184b 100644
--- a/Source/core/layout/LayoutTextControlSingleLine.cpp
+++ b/Source/core/layout/LayoutTextControlSingleLine.cpp
@@ -297,7 +297,7 @@ LayoutRect LayoutTextControlSingleLine::controlClipRect(const LayoutPoint& addit
     return clipRect;
 }
 
-float LayoutTextControlSingleLine::getAvgCharWidth(AtomicString family)
+float LayoutTextControlSingleLine::getAvgCharWidth(const AtomicString& family) const
 {
     // Since Lucida Grande is the default font, we want this to match the width
     // of MS Shell Dlg, the default font for textareas in Firefox, Safari Win and