Move |PlatformFont::GetStringWidth()| implementation up into font.cc directly.

ui/gfx/platform_font_mac.h

 // Copyright (c) 2011 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 UI_GFX_PLATFORM_FONT_MAC_H_
 #define UI_GFX_PLATFORM_FONT_MAC_H_
 #pragma once
 
 #include "base/compiler_specific.h"
 #include "ui/gfx/platform_font.h"
 
 namespace gfx {
 
 class PlatformFontMac : public PlatformFont {
  public:
   PlatformFontMac();
   explicit PlatformFontMac(const Font& other);
   explicit PlatformFontMac(NativeFont native_font);
   PlatformFontMac(const std::string& font_name,
                   int font_size);
 
   // Overridden from PlatformFont:
   virtual Font DeriveFont(int size_delta, int style) const OVERRIDE;
   virtual int GetHeight() const OVERRIDE;
   virtual int GetBaseline() const OVERRIDE;
   virtual int GetAverageCharacterWidth() const OVERRIDE;
-  virtual int GetStringWidth(const string16& text) const OVERRIDE;
   virtual int GetExpectedTextWidth(int length) const OVERRIDE;
   virtual int GetStyle() const OVERRIDE;
   virtual std::string GetFontName() const OVERRIDE;
   virtual int GetFontSize() const OVERRIDE;
   virtual NativeFont GetNativeFont() const OVERRIDE;
 
  private:
   PlatformFontMac(const std::string& font_name, int font_size, int style);
   virtual ~PlatformFontMac() {}
 
@@ skipping 11 matching lines @@
 
   // Cached metrics, generated at construction
   int height_;
   int ascent_;
   int average_width_;
 };
 
 }  // namespace gfx
 
 #endif  // UI_GFX_PLATFORM_FONT_MAC_H_