[signin] Update Reauth dialog frame color for the dark theme

In the dark mode, the reauth dialog frame color should match the
contents background color, as it does in the light mode.

Bug: 1045515
Change-Id: I701e8ece59c9a416f6fd4cf3e1c57e82e418c03e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2210502Reviewed-by: David Roger <droger@chromium.org>
Commit-Queue: Alex Ilin <alexilin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#771556}

chrome/browser/ui/views/profiles/signin_view_controller_delegate_views.cc

@@ -276,9 +276,12 @@ void SigninViewControllerDelegateViews::DisplayModal() {
       NOTREACHED() << "Unsupported dialog modal type " << dialog_modal_type_;
   }
   if (should_show_close_button_) {
+    SkColor border_color = GetNativeTheme()->ShouldUseDarkColors()
+                               ? gfx::kGoogleGrey900
+                               : SK_ColorWHITE;
     GetBubbleFrameView()->SetBubbleBorder(std::make_unique<views::BubbleBorder>(
         views::BubbleBorder::NONE, views::BubbleBorder::BIG_SHADOW,
-        SK_ColorWHITE));
+        border_color));
   }
 
   content_view_->RequestFocus();