scanning: Set darker scan preview border

Screenshot: http://screen/6nEEYaC23tGoJAh

Bug: 1059779
Change-Id: Ifb0c27b9131acde53029acc4541349c6c2e41423
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2618474Reviewed-by: Jesse Schettler <jschettler@chromium.org>
Reviewed-by: Zentaro Kavanagh <zentaro@chromium.org>
Commit-Queue: Gavin Williams <gavinwill@chromium.org>
Cr-Commit-Position: refs/heads/master@{#842227}

chromeos/components/scanning/resources/scan_preview.html

@@ -52,7 +52,7 @@
   }
 
   .preview-item {
-    border: 1px solid var(--google-grey-200);
+    border: 1px solid var(--google-grey-300);
     border-radius: 4px;
     /* Subtract 2px for the border. */
     width: calc(100% - 2px);