linux: Get font DPI from GTK.

ui/gfx/linux_font_delegate.h

 // Copyright 2013 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_LINUX_FONT_DELEGATE_H_
 #define UI_GFX_LINUX_FONT_DELEGATE_H_
 
 #include <string>
 
 #include "ui/gfx/font_render_params.h"
@@ skipping 24 matching lines @@
   // What sort of text hinting should we apply?
   virtual FontRenderParams::Hinting GetHintingStyle() const = 0;
 
   // What sort of subpixel rendering should we perform.
   virtual FontRenderParams::SubpixelRendering
       GetSubpixelRenderingStyle() const = 0;
 
   // Returns the Pango description for the default UI font. The format matches
   // that returned by pango_font_description_to_string().
   virtual std::string GetDefaultFontDescription() const = 0;
+
+  // Returns the resolution (as pixels-per-inch) that should be used to convert
+  // font sizes between points and pixels. -1 is returned if the DPI is unset.

[msw, 2014/07/18 03:39:33]

nit: return 96 instead of -1 here (vs in GetPangoDPI)?

[Daniel Erat, 2014/07/18 04:20:25]

i thought about this, but it seemed a bit cleaner to me to distinguish between
the unset and set-to-96 cases. gtk itself doesn't say what the default should
be: "Resolution for Xft, in 1024 * dots/inch. -1 to use default value."
(https://developer.gnome.org/gtk3/stable/GtkSettings.html#GtkSettings--gtk-xft...)

+  virtual double GetFontDPI() const = 0;
 };
 
 }  // namespace gfx
 
 #endif  // UI_GFX_LINUX_FONT_DELEGATE_H_