Fixed wrong background color of default browser prompt dialog

fix brave/brave-browser#23422

This is regression from below upstream change.
Fixed by specifying color from BubbleBorder ctor.
By default, kColorDialogBackground will be picked.

https://chromium-review.googlesource.com/c/chromium/src/+/3600776
Make BubbleBorder prefer themed colors to SkColors.

This also fixes the signin_view_controller_delegate_views.cc code to use
a color ID instead of a locally-hardcoded SkColor.

browser/ui/views/brave_default_browser_dialog_view.cc

@@ -131,8 +131,7 @@ BraveDefaultBrowserDialogView::CreateNonClientFrameView(views::Widget* widget) {
   const views::BubbleBorder::Shadow kShadow =
       views::BubbleBorder::DIALOG_SHADOW;
   std::unique_ptr<views::BubbleBorder> border =
-      std::make_unique<views::BubbleBorder>(views::BubbleBorder::FLOAT, kShadow,
-                                            gfx::kPlaceholderColor);
+      std::make_unique<views::BubbleBorder>(views::BubbleBorder::FLOAT, kShadow);
   if (GetParams().round_corners)
     border->SetCornerRadius(GetCornerRadius());
   frame->SetFootnoteView(DisownFootnoteView());