Add window.IN_TEST in gallery/js/background.js.

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 cd9595c36b12ca506e1d39a3cd24340b53c524e6..7bc6804890353830f9c0171263bb0ebea4f53f16 100644
--- a/ui/file_manager/externs/gallery_foreground.js
+++ b/ui/file_manager/externs/gallery_foreground.js
@@ -36,6 +36,14 @@ function FileManager() {}
 Window.prototype.appID;
 
 /**
+ * A global flag which indicates whether it is in tests or not. This is set in
+ * gallery/js/background.js.
+ *
+ * @type {boolean}
+ */
+Window.prototype.IN_TEST;
+
+/**
  * @type {string}
  */
 Entry.prototype.cachedUrl;