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

window.IN_TEST is not defined in gallery app. It caused a warning of closure compiler, and IN_TEST flag should be set true also in tests of gallery app.

BUG=433728
TEST=GYP_GENERATORS=ninja tools/gyp/gyp --depth . ui/file_manager/gallery/js/compiled_resources.gyp && ninja -C out/Default
out/Release/browser_tests --gtest_filter=*GalleryBrowserTest*

Review URL: https://codereview.chromium.org/817233002

Cr-Commit-Position: refs/heads/master@{#309402}

ui/file_manager/externs/gallery_foreground.js

@@ -35,6 +35,14 @@ 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}
  */

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) {