scanning: Fix shadow height on scanned image

This change fixes the gap between the bottom of the scanned image border
and its focus shadow.

Before: http://screen/6tdVfoSCvnEy72p
After: http://screen/5LVWsQbm3NEdkAy

Bug: 1059779
Change-Id: I31c30b9cc940b8b4394da71fc4b3287dc43a4527
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2618557Reviewed-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@{#841823}

chromeos/components/scanning/resources/scan_preview.html

@@ -58,6 +58,10 @@
     width: calc(100% - 2px);
   }
 
+  .scanned-image {
+    display: block;
+  }
+
   /* Add top margin to all but the first scanned image. */
   .scanned-image:nth-of-type(n+2) {
     margin-top: 8px;