Use references in public EventSender functions

Use references in public EventSender functions
https://bugs.webkit.org/show_bug.cgi?id=136463

Reviewed by Dan Bates.

Passing nullptr to EventSender shouldn't be allowed.

No new tests because there is no behavior change.

* dom/EventSender.h:
(WebCore::EventSender::hasPendingEvents):
(WebCore::EventSender<T>::dispatchEventSoon):
(WebCore::EventSender<T>::cancelEvent):
(WebCore::EventSender<T>::dispatchPendingEvents):
* html/HTMLLinkElement.cpp:
(WebCore::HTMLLinkElement::~HTMLLinkElement):
(WebCore::HTMLLinkElement::notifyLoadedSheetAndAllCriticalSubresources):
* html/HTMLStyleElement.cpp:
(WebCore::HTMLStyleElement::~HTMLStyleElement):
(WebCore::HTMLStyleElement::notifyLoadedSheetAndAllCriticalSubresources):
* loader/ImageLoader.cpp:
(WebCore::ImageLoader::~ImageLoader):
(WebCore::ImageLoader::setImageWithoutConsideringPendingLoadEvent):
(WebCore::ImageLoader::updateFromElement):
(WebCore::ImageLoader::notifyFinished):
(WebCore::ImageLoader::dispatchPendingBeforeLoadEvent):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@173190 268f45cc-cd09-0410-ab3c-d52691b4dbfc