Uses and returns the fractional width in text eliding

ui/gfx/platform_font_mac.mm

 // Copyright (c) 2012 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.
 
 #include "ui/gfx/platform_font_mac.h"
 
 #include <Cocoa/Cocoa.h>
 
 #include "base/basictypes.h"
 #include "base/mac/scoped_nsobject.h"
 #include "base/strings/sys_string_conversions.h"
 #include "base/strings/utf_string_conversions.h"
 #include "ui/gfx/canvas.h"
 #include "ui/gfx/font.h"
+#include "ui/gfx/font_list.h"
 
 namespace gfx {
 
 ////////////////////////////////////////////////////////////////////////////////
 // PlatformFontMac, public:
 
 PlatformFontMac::PlatformFontMac() {
   font_size_ = [NSFont systemFontSize];
   style_ = gfx::Font::NORMAL;
   NSFont* system_font = [NSFont systemFontOfSize:font_size_];
@@ skipping 40 matching lines @@
 
 int PlatformFontMac::GetAverageCharacterWidth() const {
   return average_width_;
 }
 
 int PlatformFontMac::GetStringWidth(const base::string16& text) const {
   return Canvas::GetStringWidth(text,
                                 Font(const_cast<PlatformFontMac*>(this)));
 }
 
+float PlatformFontMac::GetStringWidthF(const base::string16& text) const {
+  return Canvas::GetStringWidthF(
+      text, FontList(Font(const_cast<PlatformFontMac*>(this))));
+}
+
 int PlatformFontMac::GetExpectedTextWidth(int length) const {
   return length * average_width_;
 }
 
 int PlatformFontMac::GetStyle() const {
   return style_;
 }
 
 std::string PlatformFontMac::GetFontName() const {
   return font_name_;
@@ skipping 65 matching lines @@
   return new PlatformFontMac(native_font);
 }
 
 // static
 PlatformFont* PlatformFont::CreateFromNameAndSize(const std::string& font_name,
                                                   int font_size) {
   return new PlatformFontMac(font_name, font_size);
 }
 
 }  // namespace gfx