Add a new GetSystemColor method to native theme.

ui/gfx/native_theme.h

Index: ui/gfx/native_theme.h
diff --git a/ui/gfx/native_theme.h b/ui/gfx/native_theme.h
index cc9aa35e4049bcdb8eabadaa4158b330415a96f0..0db8bdf3032b15510ebb0a5af4d6c56f79ee714d 100644
--- a/ui/gfx/native_theme.h
+++ b/ui/gfx/native_theme.h
@@ -19,11 +19,13 @@ 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
@@ -212,6 +214,15 @@ class UI_EXPORT NativeTheme {
                           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.