Dark mode: add color for footnote view separator

Bug: 918194
Change-Id: I6572c33b8e1a607b612b6dbf5c7a5a9f369862d6
Reviewed-on: https://chromium-review.googlesource.com/c/1394739Reviewed-by: Michael Wasserman <msw@chromium.org>
Commit-Queue: Leonard Grey <lgrey@chromium.org>
Cr-Commit-Position: refs/heads/master@{#619777}

ui/views/bubble/footnote_container_view.cc

@@ -54,7 +54,10 @@ FootnoteContainerView::FootnoteContainerView(const gfx::Insets& margins,
   SetLayoutManager(
       std::make_unique<BoxLayout>(BoxLayout::kVertical, margins, 0));
   SetCornerRadius(corner_radius);
-  SetBorder(CreateSolidSidedBorder(1, 0, 0, 0, gfx::kGoogleGrey200));
+  SetBorder(CreateSolidSidedBorder(1, 0, 0, 0,
+                                   GetNativeTheme()->SystemDarkModeEnabled()
+                                       ? gfx::kGoogleGrey900
+                                       : gfx::kGoogleGrey200));
   AddChildView(child_view);
   SetVisible(child_view->visible());
 }