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..4932bbb0682bf15f3f7dae062f7922ae46c59d52 100644
--- a/ui/gfx/native_theme.h
+++ b/ui/gfx/native_theme.h
@@ -14,16 +14,23 @@ class SkCanvas;
 
 namespace gfx {
 
+namespace {
+// Returned by GetSystemColor() in response to an invalid ColorId.
+const SkColor kColorYouShouldNeverSee = SkColorSetRGB(255, 0, 128);

[James Cook, 2011/11/19 00:28:14]

I wouldn't do this in a header file, even though the color ends up as an
unsigned int.  It just looks too much like you're initializing an object.  We're
super-paranoid about global object init/destruct around here.

I would either just leave copies of the color in each .cc file that needs it, or
create a one-line utility function in the base implementation to get it.

[benrg, 2011/11/21 18:32:40]

It seems silly to add a public method for code that should never be hit, so I
just duplicated the color.

+}
+
 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
@@ -212,6 +219,15 @@ class UI_EXPORT NativeTheme {
                           unsigned active_color,
                           unsigned track_color) const;
 
+  // Colors for GetSystemColor().
+  enum ColorId {
+    kDialogBackgroundColorId
+    // 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.