scanning: Change scrollbar behavior

Ensure the scrollbar is always visible when scanned images extend beyond
the app window.

Bug: 1059779
Change-Id: I6aca5b1ffa478e1cfff4d5f081e92f79a86eb535
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2552214
Commit-Queue: Jesse Schettler <jschettler@chromium.org>
Reviewed-by: Zentaro Kavanagh <zentaro@chromium.org>
Cr-Commit-Position: refs/heads/master@{#829913}

chromeos/components/scanning/resources/scan_preview.html

@@ -34,6 +34,16 @@
     overflow-y: scroll;
   }
 
+  .preview::-webkit-scrollbar {
+    -webkit-appearance: none;
+    width: 4px;
+  }
+
+  .preview::-webkit-scrollbar-thumb {
+    background-color: rgba(0, 0, 0, 0.5);
+    border-radius: 4px;
+  }
+
   .preview-item {
     border: 1px solid var(--google-grey-200);
     border-radius: 4px;