ImageLoader can still dispatch beforeload events for ImageDocuments

ImageLoader can still dispatch beforeload events for ImageDocuments
https://bugs.webkit.org/show_bug.cgi?id=86658
<rdar://problem/11465863>

Reviewed by Brady Eidson.

Prevent flags regarding sending beforeload events from being set on ImageDocuments.

No new tests; testing framework doesn't allow for testing ImageDocuments with injected JavaScript.

* loader/ImageLoader.cpp:
(WebCore::ImageLoader::updateFromElement):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@117336 268f45cc-cd09-0410-ab3c-d52691b4dbfc