Split the private webstore install API into two parts....

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

+// Copyright (c) 2010 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+var extension_id = "ldnnhddmnhbkjipkidpdiheffobcpfmf";
+
+var assertEq = chrome.test.assertEq;
+var assertNoLastError = chrome.test.assertNoLastError;
+var succeed = chrome.test.succeed;
+
+// Calls |callback| with true/false indicating whether an item with an id of
+// extension_id is installed.
+function checkInstalled(callback) {
+  chrome.management.getAll(function(extensions) {
+    var found = false;
+    callback(extensions.some(function(ext) {
+      return ext.id == extension_id;
+    }));
+  });
+}