Add a flag that lets the webstore show a different UI on app install.

chrome/test/data/extensions/api_test/webstore_private/common.js

Index: chrome/test/data/extensions/api_test/webstore_private/common.js
diff --git a/chrome/test/data/extensions/api_test/webstore_private/common.js b/chrome/test/data/extensions/api_test/webstore_private/common.js
index 933b032d22077a55e2f9a34fb75999cf2b4a2e21..358ced02e25d7aa45c432dd3eeb52cbb3fd57236 100644
--- a/chrome/test/data/extensions/api_test/webstore_private/common.js
+++ b/chrome/test/data/extensions/api_test/webstore_private/common.js
@@ -2,9 +2,12 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// The id of the extension we're using for install tests.
+// The id of an extension we're using for install tests.
 var extensionId = "enfkhcelefdadlmkffamgdlgplcionje";
 
+// The id of an app we're using for install tests.
+var appId = "iladmdjkfniedhfhcfoefgojhgaiaccc";
+
 var assertEq = chrome.test.assertEq;
 var assertNoLastError = chrome.test.assertNoLastError;
 var callbackFail = chrome.test.callbackFail;