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

chrome/test/data/extensions/api_test/extension_gallery_install/test.html

 <script>
 // This tests that the management install and uninstall functions work
 // properly when called by the gallery. Additionally, it implicitly tests
 // that the install() function, when called from the gallery, avoids the
 // dangerous download prompt and the extension permissions install prompt.
 // If either were to appear, this test wouldn't complete.
 // Note that for the purposes of this test the gallery url is "www.a.com"
 // which is set in extension_gallery_install_apitest.cc.
 
 var id = "ldnnhddmnhbkjipkidpdiheffobcpfmf";
 
 function checkInstalled(callback) {
   chrome.management.getAll(function(extensions) {
     var found = false;
     extensions.forEach(function(extension) {
       if (id == extension.id)
         found = true;
     });
-    
+
     callback(found);
   });
 }
 
 // Make sure our "gallery" extension isn't yet installed.
 checkInstalled(function(installed) {
   chrome.test.assertEq(false, installed);
 
   // Install "gallery" extensions.
-  chrome.management.onInstalled.addListener(function(info) {  
+  chrome.management.onInstalled.addListener(function(info) {
     chrome.test.assertEq(id, info.id);
     console.log("Installed " + info.id);
-    
+
     // Double check it is installed.
     checkInstalled(function(installed) {
       chrome.test.assertEq(true, installed);
 
       // Now uninstall
       chrome.management.onUninstalled.addListener(function() {
         // And check that it's gone.
         checkInstalled(function(installed) {
           chrome.test.assertEq(false, installed);
-          
+
           chrome.test.succeed();
         });
       });
 
       console.log("Uninstalling...");
       chrome.management.uninstall(id, function() {
         chrome.test.assertNoLastError();
       });
     });
   });
-  chrome.webstorePrivate.install(id, function() {
+  chrome.webstorePrivate.beginInstall(id, function() {
+    if (window.location.href.indexOf("expect_error=true") > 0) {
+      chrome.test.assertEq(chrome.extension.lastError.message,
+          "This function must be called during a user gesture");
+      chrome.test.succeed();
+      return;
+    }
     chrome.test.assertNoLastError();
+    chrome.webstorePrivate.completeInstall(id, function() {

[Erik does not do reviews, 2010/11/09 16:32:21]

seems like you need a couple of tests that call completeInstall without
beginInstall or with a failed beginInstall.

[asargent_no_longer_on_chrome, 2010/11/10 00:23:48]

I've simplified this test and added others to cover the cases you suggested. 

+      chrome.test.assertNoLastError();
+    });
   });
   console.log("Installing...");
 });
 </script>