gfx: Make conversions from Size to SizeF be explicit.

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 <cmath>
+
 #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_render_params.h"
 
@@ skipping 184 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