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

ui/file_manager/gallery/js/background.js

Index: ui/file_manager/gallery/js/background.js
diff --git a/ui/file_manager/gallery/js/background.js b/ui/file_manager/gallery/js/background.js
index 3463f836145bbbf493a17b91de99a4d2c8a45c55..e65a737050bfcf4a9fde155d4aa189cf8666e314 100644
--- a/ui/file_manager/gallery/js/background.js
+++ b/ui/file_manager/gallery/js/background.js
@@ -222,6 +222,10 @@ if (chrome.app.runtime) {
 // If is is run in the browser test, wait for the test resources are installed
 // as a component extension, and then load the test resources.
 if (chrome.test) {
+  // Sets a global flag that we are in tests, so other components are aware of
+  // it.
+  window.IN_TEST = true;
+
   /** @type {string} */
   window.testExtensionId = 'ejhcmmdhhpdhhgmifplfmjobgegbibkn';
   chrome.runtime.onMessageExternal.addListener(function(message) {