ヘルプのコード
String parentWindowHandle = browser.getWindowHandle(); // save the current window handle.
WebDriver popup = null;
Iterator<String> windowIterator = browser.getWindowHandles();
while(windowIterator.hasNext()) {
String windowHandle = windowIterator.next();
popup = browser.switchTo().window(windowHandle);
if (popup.getTitle().equals("Google") {
break;
}
}