Make the search box smaller when inactive and grow when focused.

chrome/browser/resources/file_manager/css/file_manager.css

Index: chrome/browser/resources/file_manager/css/file_manager.css
diff --git a/chrome/browser/resources/file_manager/css/file_manager.css b/chrome/browser/resources/file_manager/css/file_manager.css
index d88cee3727ad3474f1660c0de3fe419007791ed3..5d28f7fbf1adf7c51ad2154022adf112ddffe394 100644
--- a/chrome/browser/resources/file_manager/css/file_manager.css
+++ b/chrome/browser/resources/file_manager/css/file_manager.css
@@ -451,6 +451,7 @@ div.root-eject:hover {
 }
 
 #search-box {
+  -webkit-transition: width 600ms;
   background: -webkit-image-set(
     url('../images/files/ui/icon_search.png') 1x,
     url('../images/files/ui/2x/icon_search.png') 2x);
@@ -460,6 +461,11 @@ div.root-eject:hover {
   height: 25px;
   margin-right: 10px;
   padding-left: 22px;
+  width: 150px;
+}
+
+#search-box:focus {
+  width: 240px;
 }
 
 #search-box,