Change Files app's banner controller into a banners UI class.

ui/file_manager/file_manager/foreground/js/ui/file_manager_ui.js

Index: ui/file_manager/file_manager/foreground/js/ui/file_manager_ui.js
diff --git a/ui/file_manager/file_manager/foreground/js/ui/file_manager_ui.js b/ui/file_manager/file_manager/foreground/js/ui/file_manager_ui.js
index 6fb84305f8122e1911c9823ac2909b723f3238c6..236d439bfaf4d0f9b756fdc800708ccc55e060d4 100644
--- a/ui/file_manager/file_manager/foreground/js/ui/file_manager_ui.js
+++ b/ui/file_manager/file_manager/foreground/js/ui/file_manager_ui.js
@@ -226,6 +226,12 @@ function FileManagerUI(element, launchParam) {
   };
 
   /**
+   * Banners in the file list.
+   * @type {Banners}
+   */
+  this.banners = null;
+
+  /**
    * Dialog footer.
    * @type {!DialogFooter}
    */
@@ -331,6 +337,15 @@ FileManagerUI.prototype.initDirectoryTree = function(directoryTree) {
 };
 
 /**
+ * TODO(mtomasz): Merge the method into initAdditionalUI if possible.
+ * @param {!Banners} banners
+ */
+FileManagerUI.prototype.initBanners = function(banners) {
+  this.banners = banners;
+  this.banners.addEventListener('relayout', this.relayout.bind(this));
+};
+
+/**
  * Relayouts the UI.
  */
 FileManagerUI.prototype.relayout = function() {