Eliminate hardcoded SkColorSet* from message_view

Per guidance, this changes the color used in message_view to
kColorId_UnfocusedBorderColor.

Bug: 1056950
Change-Id: Ibe265a94c5552e01f7d5cbd330e42a940854f164
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2117291Reviewed-by: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Justin DeWitt <dewittj@chromium.org>
Reviewed-by: Evan Stade <estade@chromium.org>
Commit-Queue: Maria Villarreal <mavill@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#753467}

ui/message_center/views/message_view.cc

@@ -460,7 +460,8 @@ bool MessageView::ShouldShowControlButtons() const {
 
 void MessageView::SetNestedBorderIfNecessary() {
   if (is_nested_) {
-    SkColor border_color = SkColorSetARGB(0x1F, 0x0, 0x0, 0x0);
+    SkColor border_color = GetNativeTheme()->GetSystemColor(
+        ui::NativeTheme::kColorId_UnfocusedBorderColor);
     SetBorder(views::CreateRoundedRectBorder(
         kNotificationBorderThickness, kNotificationCornerRadius, border_color));
   }