Add externs for gallery.

ui/file_manager/externs/gallery_foreground.js

Index: ui/file_manager/externs/gallery_foreground.js
diff --git a/ui/file_manager/externs/gallery_foreground.js b/ui/file_manager/externs/gallery_foreground.js
index 474ca507caf4039235607b21217772ae7b8e6241..cd9595c36b12ca506e1d39a3cd24340b53c524e6 100644
--- a/ui/file_manager/externs/gallery_foreground.js
+++ b/ui/file_manager/externs/gallery_foreground.js
@@ -18,3 +18,24 @@ function BackgroundComponents() {
    */
   this.volumeManager;
 }
+
+/**
+ * File Manager. This definition is required by
+ * ui/file_manager/file_manager/foreground/js/ui/file_manager_dialog_base.js.
+ *
+ * @constructor
+ * @struct
+ */
+function FileManager() {}
+
+/**
+ * This definition is required by
+ * ui/file_manager/file_manager/common/js/util.js.
+ * @type {string}
+ */
+Window.prototype.appID;
+
+/**
+ * @type {string}
+ */
+Entry.prototype.cachedUrl;