MenuButton: support Mac look & feel

ui/views/style/platform_style.h

Index: ui/views/style/platform_style.h
diff --git a/ui/views/style/platform_style.h b/ui/views/style/platform_style.h
index e6916176bf4de6958f2518b8231de018547dd145..77677922099dfd54c74c80caee6c2881127cf2eb 100644
--- a/ui/views/style/platform_style.h
+++ b/ui/views/style/platform_style.h
@@ -52,6 +52,26 @@ class VIEWS_EXPORT PlatformStyle {
   static std::unique_ptr<LabelButtonBorder> CreateLabelButtonBorder(
       Button::ButtonStyle style);
 
+  // Creates an ImageSkia containing the image to use for the menubutton arrow.
+  static gfx::ImageSkia CreateMenuButtonArrow(bool is_enabled);
+
+  // Create the appropriate background for a MenuButton.
+  static std::unique_ptr<Background> CreateMenuButtonBackground();
+
+  // Create the appropriate border for a MenuButton.
+  static std::unique_ptr<FocusableBorder> CreateMenuButtonBorder();
+
+  // A combobox or menubutton looks like this:
+  // ------------------------------------
+  // | text field ............. | arrow |
+  // ------------------------------------
+  // These functions return the width of the arrow part. This includes the arrow
+  // image itself as well as any padding on either side of it. On Mac, this is
+  // the width that will be filled by the arrow button's background, so text
+  // must not encroach upon it.
+  static int GetComboboxArrowWidth(Combobox::Style style);

[tapted, 2016/04/21 06:06:46]

At this point "shoulder" is really starting to grow on me :)

[Elly Fong-Jones, 2016/04/21 15:30:27]

Done.

+  static int GetMenuButtonArrowWidth();
+
   // Creates the default scrollbar for the given orientation.
   static std::unique_ptr<ScrollBar> CreateScrollBar(bool is_horizontal);