[actionbar] Make search box collapse smoother

Adds a transition for the background-color property on the search box
wrapper so that it fades out while the search text entry field is
collapsing to stop the sudden change of color when the text field starts
to collapse.

Bug: 1075941
Change-Id: I25d38c2db817b2585b8c86c046e5d88e041ec30a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2210297Reviewed-by: Noel Gordon <noel@chromium.org>
Commit-Queue: Alex Danilo <adanilo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#771464}

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

@@ -734,6 +734,7 @@ html[dir='rtl'] .dialog-header button.icon-button > files-toggle-ripple {
   align-items: center;
   border-radius: 4px;
   height: 40px;
+  transition: background-color 200ms ease;
 }
 
 .dialog-header.files-ng #search-wrapper.has-cursor,