We should consider implementing testRunner.setCanOpenWindows() to support writing layout tests when the embedding client returns a null pointer from the ChromeClient::createWindow() callback. One example of such a test is LayoutTests/plugins/get-url-with-blank-target.html: <http://trac.webkit.org/browser/trunk/LayoutTests/plugins/get-url-with-blank-target.html?rev=210022>. Maybe TestWebKitAPI tests would be a better fit for layout tests that want to test such behavior?