MacViews: draw combobox arrow backgrounds

ui/native_theme/native_theme.h

Index: ui/native_theme/native_theme.h
diff --git a/ui/native_theme/native_theme.h b/ui/native_theme/native_theme.h
index f4dbd690a7edffe0645b6a3bca0472f367c749dd..af3d8085770d9ef9cd3d13158e886d7b4516a616 100644
--- a/ui/native_theme/native_theme.h
+++ b/ui/native_theme/native_theme.h
@@ -45,6 +45,7 @@ class NATIVE_THEME_EXPORT NativeTheme {
   // The part to be painted / sized.
   enum Part {
     kComboboxArrow,
+    kComboboxArrowBackground,

[tapted, 2016/03/21 07:09:04]

My first thought (I don't know if it's a good one or not) is to make this part
of the border. That's what's done currently for buttons.

that could let us get away with fewer (or no?) changes to combobox.cc -- maybe
we just need to ensure isets are dealt with properly.

[Elly Fong-Jones, 2016/03/21 18:53:56]

Hm. I don't really like the idea of having part of the control (the arrow
button) be drawn outside/over the "border" of the control. We'd also basically
need a custom "combobox mac border" class to implement this, I think. One other
design wrinkle is that the border is usually painted last, but if we relied on
the border to draw the arrow background, we'd need to draw the arrow after that
part of the border.

[tapted, 2016/03/22 11:31:09]

oops - missed this. Borders is largely how more complex buttons are currently
done. E.g. the stuff in ui/views/controls/button/label_button_border.h, 
Gtk2UI::CreateNativeBorder/GtkButtonPainter/GtkButtonImageSource and
ui/views/style/mac/dialog_button_border_mac.h

View::OnPaint defaults to background, then border, (then back into View::Paint
to paint children). If something overrides View::OnPaint (like combobox!) it has
can control the order - I don't think there's a problem problem moving
OnPaintBorder up (or even calling super::OnPaint). In fact, if Combobox used a
child views::Label for its text the way most other controls do it would have no
option but to paint its border before the text.

(and yes, combobox should perhaps also use a views::Label for its text, but
that's orthogonal...)

     kCheckbox,
     kInnerSpinButton,
     kMenuList,