Abstract ImageEventSender into a general purpose EventSender

Abstract ImageEventSender into a general purpose EventSender
https://bugs.webkit.org/show_bug.cgi?id=78840

Reviewed by Adam Barth.

Abstract the functionality in ImageEventSender so that it can be used again.
This functionality may be useful in fixing WebKit Bug #38995.

No functionality was changed; no new tests.

* GNUmakefile.list.am: Added EventSender.h.
* Target.pri: Ditto.
* WebCore.gypi: Ditto.
* WebCore.vcproj/WebCore.vcproj: Ditto.
* WebCore.xcodeproj/project.pbxproj: Ditto.
* dom/EventSender.h: Added.
(WebCore):
(EventSender):
(WebCore::EventSender::eventType):
(WebCore::EventSender::hasPendingEvents):
(WebCore::EventSender::timerFired):
(WebCore::::EventSender):
(WebCore::::dispatchEventSoon):
(WebCore::::cancelEvent):
(WebCore::::dispatchPendingEvents):
* loader/ImageLoader.cpp: Modified to use EventSender.
(WebCore):
(WebCore::ImageLoader::dispatchPendingEvent): Added; called by EventSender when the ImageLoader
should dispatch a pending BeforeLoad or Load event.
* loader/ImageLoader.h:
(WebCore):
(ImageLoader):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@108561 268f45cc-cd09-0410-ab3c-d52691b4dbfc