Fixed font height on OS X. Also added some unittests.

app/gfx/font.h

 // Copyright (c) 2006-2008 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 APP_GFX_FONT_H_
 #define APP_GFX_FONT_H_
 
 #include "build/build_config.h"
 
 #include <string>
@@ skipping 55 matching lines @@
   }
 
   // Returns a new Font derived from the existing font.
   // size_delta is the size to add to the current font. See the single
   // argument version of this method for an example.
   // The style parameter specifies the new style for the font, and is a
   // bitmask of the values: BOLD, ITALIC and UNDERLINED.
   Font DeriveFont(int size_delta, int style) const;
 
   // Returns the number of vertical pixels needed to display characters from
-  // the specified font.
+  // the specified font.  This may include some leading, i.e. height may be
+  // greater than just ascent + descent.  Specifically, the Windows and Mac
+  // implementations include leading and the Linux one does not.  This may
+  // need to be revisited in the future.
   int height() const;
 
   // Returns the baseline, or ascent, of the font.
   int baseline() const;
 
   // Returns the average character width for the font.
   int ave_char_width() const;
 
   // Returns the number of horizontal pixels needed to display the specified
   // string.
@@ skipping 170 matching lines @@
   int height_;
   int ascent_;
   int avg_width_;
 #endif
 
 };
 
 }  // namespace gfx
 
 #endif  // APP_GFX_FONT_H_