Add a new GetSystemColor method to native theme.

ui/gfx/native_theme.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_NATIVE_THEME_H_
 #define UI_GFX_NATIVE_THEME_H_
 #pragma once
 
 #include "third_party/skia/include/core/SkColor.h"
 #include "ui/base/ui_export.h"
 #include "ui/gfx/native_widget_types.h"
 
 class SkCanvas;
 
 namespace gfx {
 
 class Rect;
 class Size;
 
 // This class supports drawing UI controls (like buttons, text fields, lists,
 // comboboxes, etc) that look like the native UI controls of the underlying
-// platform, such as Windows or Linux.
+// platform, such as Windows or Linux. It also supplies default colors for
+// dialog box backgrounds, etc., which are obtained from the system theme where
+// possible.
 //
 // The supported control types are listed in the Part enum.  These parts can be
-// in any state given by the State enum, where the actual definititon of the
-// state is part-specific.
+// in any state given by the State enum, where the actual definition of the
+// state is part-specific. The supported colors are listed in the ColorId enum.
 //
 // Some parts require more information than simply the state in order to be
 // drawn correctly, and this information is given to the Paint() method via the
 // ExtraParams union.  Each part that requires more information has its own
 // field in the union.
 //
 // NativeTheme also supports getting the default size of a given part with
 // the GetPartSize() method.
 class UI_EXPORT NativeTheme {
  public:
@@ skipping 168 matching lines @@
                      Part part,
                      State state,
                      const gfx::Rect& rect,
                      const ExtraParams& extra) const = 0;
 
   // Supports theme specific colors.
   void SetScrollbarColors(unsigned inactive_color,
                           unsigned active_color,
                           unsigned track_color) const;
 
+  // Colors for GetSystemColor().
+  enum ColorId {
+    kColorId_DialogBackground
+    // TODO(benrg): move other hardcoded colors here.
+  };
+
+  // Return a color from the system theme.
+  virtual SkColor GetSystemColor(ColorId color_id) const = 0;
+
   // Returns a shared instance of the native theme.
   // The returned object should not be deleted by the caller.  This function
   // is not thread safe and should only be called from the UI thread.
   // Each port of NativeTheme should provide its own implementation of this
   // function, returning the port's subclass.
   static const NativeTheme* instance();
 
  protected:
   NativeTheme() {}
   virtual ~NativeTheme() {}
 
   static unsigned int thumb_inactive_color_;
   static unsigned int thumb_active_color_;
   static unsigned int track_color_;
 
   DISALLOW_COPY_AND_ASSIGN(NativeTheme);
 };
 
 }  // namespace gfx
 
 #endif  // UI_GFX_NATIVE_THEME_H_