[M115 Merge] Customize Chrome: Fix not scrolling to cards

`.sp-card` has a CSS property value of `display: contents` by default
which prevents the scroll from occurring.

(cherry picked from commit 2744927)

Fixed: 1449336
Change-Id: I98e22cd3389a27e6609de3a851287d0c52af43a7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4571342
Commit-Queue: John Lee <johntlee@chromium.org>
Reviewed-by: Riley Tatum <rtatum@google.com>
Cr-Original-Commit-Position: refs/heads/main@{#1150722}
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4590551
Cr-Commit-Position: refs/branch-heads/5790@{#364}
Cr-Branched-From: 1d71a33-refs/heads/main@{#1148114}

chrome/browser/resources/side_panel/customize_chrome/app.html

@@ -11,6 +11,10 @@
     padding: var(--sp-body-padding) 0;
   }
 
+  .section {
+    display: block;
+  }
+
   sp-heading {
     margin: 8px 16px;
   }