Fix realbox border in forced-colors mode

THe box-shadow property isn't shown in forced colors mode,
so give the realbox a real border instead.

Bug: 1166383
Change-Id: I8a1aa58d965a0fed1a69b0210273a984c8478a5d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2628077Reviewed-by: Esmael Elmoslimany <aee@chromium.org>
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843853}

chrome/browser/resources/new_tab_page/realbox.html

@@ -6,6 +6,12 @@
     height: var(--ntp-realbox-height);
   }
 
+  @media (forced-colors: active) {
+    :host {
+      border: 1px solid ActiveBorder;
+    }
+  }
+
   :host([matches-are-visible]) {
     box-shadow: none;
   }