views: reduce mac menu border margin

Mac menus don't use a margin around the contents at all - the menu items
(especially their highlights) draw hard up to the menu edge.

TBR=sky@chromium.org

Bug: 828117
Change-Id: I0165bc8987d0f7d6c1ba29660098f91ad9478984
Reviewed-on: https://chromium-review.googlesource.com/993418Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547800}

ui/views/controls/menu/menu_config_mac.mm

@@ -13,6 +13,7 @@ namespace views {
 void MenuConfig::Init() {
   font_list = gfx::FontList(gfx::Font([NSFont menuFontOfSize:0.0]));
   menu_vertical_border_size = 4;
+  menu_horizontal_border_size = 0;
   item_top_margin = item_no_icon_top_margin = 1;
   item_bottom_margin = item_no_icon_bottom_margin = 1;
   item_left_margin = 2;