Add the browser test for the new gallery.

chrome/test/data/extensions/api_test/file_manager_browsertest/gallery/background.js

Index: chrome/test/data/extensions/api_test/file_manager_browsertest/gallery/background.js
diff --git a/ui/native_theme/native_theme_observer.cc b/chrome/test/data/extensions/api_test/file_manager_browsertest/gallery/background.js
similarity index 56%
copy from ui/native_theme/native_theme_observer.cc
copy to chrome/test/data/extensions/api_test/file_manager_browsertest/gallery/background.js
index f516a762027fccab32d569f2466d82c65252c104..e8536249f66d372143efd1284066ff72ba0a748b 100644
--- a/ui/native_theme/native_theme_observer.cc
+++ b/chrome/test/data/extensions/api_test/file_manager_browsertest/gallery/background.js
@@ -2,10 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-#include "ui/native_theme/native_theme_observer.h"
+'use strict';
 
-namespace ui {
-
-NativeThemeObserver::~NativeThemeObserver() {}
-
-}  // namespace ui
+chrome.runtime.sendMessage(
+    'nlkncpkkdoccmpiclbokaimcnedabhhm',
+    {name: 'testResourceLoaded'});