Adds a webstorePrivate method for silently installing extensions.

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 358ced02e25d7aa45c432dd3eeb52cbb3fd57236..17025a42bebc09fd2291355c05e466cdfa9d3165 100644
--- a/chrome/test/data/extensions/api_test/webstore_private/common.js
+++ b/chrome/test/data/extensions/api_test/webstore_private/common.js
@@ -10,22 +10,29 @@ var appId = "iladmdjkfniedhfhcfoefgojhgaiaccc";
 
 var assertEq = chrome.test.assertEq;
 var assertNoLastError = chrome.test.assertNoLastError;
+var assertTrue = chrome.test.assertTrue;
 var callbackFail = chrome.test.callbackFail;
 var callbackPass = chrome.test.callbackPass;
 var listenOnce = chrome.test.listenOnce;
 var runTests = chrome.test.runTests;
 var succeed = chrome.test.succeed;
 
-// Calls |callback| with true/false indicating whether an item with an id of
-// extensionId is installed.
-function checkInstalled(callback) {
+// Calls |callback| with true/false indicating whether an item with the |id|
+// is installed.
+function checkItemInstalled(id, callback) {
   chrome.management.getAll(function(extensions) {
     callback(extensions.some(function(ext) {
-      return ext.id == extensionId;
+      return ext.id == id;
     }));
   });
 }
 
+// Calls |callback| with true/false indicating whether an item with an id of
+// extensionId is installed.
+function checkInstalled(callback) {
+  checkItemInstalled(extensionId, callback);
+}
+
 var cachedIcon = null;
 var img = null;