macviews: use Aura unfocused border color

The Mac unfocused border color is far lighter and looks wrong in the
MacViews Harmony UI surfaces.

Bug: 837278
Change-Id: I98807cf565e30f51ddc5486c3cc00b8cf6ae5c74
Reviewed-on: https://chromium-review.googlesource.com/1035107
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#554856}

ui/native_theme/native_theme_mac.mm

@@ -155,8 +155,6 @@ SkColor NativeThemeMac::GetSystemColor(ColorId color_id) const {
 
     case kColorId_FocusedBorderColor:
       return NSSystemColorToSkColor([NSColor keyboardFocusIndicatorColor]);
-    case kColorId_UnfocusedBorderColor:
-      return NSSystemColorToSkColor([NSColor controlColor]);
 
     default:
       break;