Add background color for search box wrapper

Uses the CSS properties 'has-cursor' and 'hide-pending' to apply a
background color to the search box wrapper when it's open.

Bug: 992823
Change-Id: I3fc27b9cac65d2cfeb5985344ad4140c8992c912
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2060215
Commit-Queue: Noel Gordon <noel@chromium.org>
Reviewed-by: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#742093}

ui/file_manager/file_manager/foreground/css/file_manager.css

@@ -671,6 +671,11 @@ html[dir='rtl'] .dialog-header button.icon-button > files-toggle-ripple {
   display: flex;
 }
 
+.dialog-header.files-ng #search-wrapper.has-cursor,
+.dialog-header.files-ng #search-wrapper.hide-pending {
+  background-color: var(--google-grey-100);
+}
+
 .dialog-header #search-box cr-input {
   --cr-input-background-color: transparent;
   --cr-input-border-radius: 0;