Reviewed by Adam Barth.

        refactor HTMLLinkElement to allow Link header implementation
        https://bugs.webkit.org/show_bug.cgi?id=51941

        This change moves the LinkRelAttribute helper struct into its own
        file in html/, since the LinkRelAttribute is used by the LinkLoader,
        and leaving it in HTMLLinkElement would have been too circular for
        my tastes.

        The new LinkLoader class contains the loader code for icons, dns
        prefetching and prefetch/prerender/subresource into a common
        LinkLoader.  This is a step in getting the Link header working in
        WebKit, as discussed in webkit-dev For more information, see
        thread "beforeload & link
        (esp rel prefetch)".

        No new tests, since this is a refactoring.

        * CMakeLists.txt:
        * GNUmakefile.list.am:
        * WebCore.gypi:
        * WebCore.pro:
        * WebCore.vcproj/WebCore.vcproj:
        * WebCore.xcodeproj/project.pbxproj:
        * html/HTMLLinkElement.cpp:
        (WebCore::HTMLLinkElement::HTMLLinkElement):
        (WebCore::HTMLLinkElement::~HTMLLinkElement):
        (WebCore::HTMLLinkElement::parseMappedAttribute):
        (WebCore::HTMLLinkElement::shouldLoadLink):
        (WebCore::HTMLLinkElement::process):
        (WebCore::HTMLLinkElement::linkLoaded):
        (WebCore::HTMLLinkElement::linkLoadingErrored):
        * html/HTMLLinkElement.h:
        * html/LinkRelAttribute.cpp: Added.
        (WebCore::LinkRelAttribute::LinkRelAttribute):
        * html/LinkRelAttribute.h: Added.
        * html/parser/HTMLPreloadScanner.cpp:
        (WebCore::HTMLNames::PreloadTask::relAttributeIsStyleSheet):
        * loader/LinkLoader.cpp: Added.
        (WebCore::LinkLoader::LinkLoader):
        (WebCore::LinkLoader::~LinkLoader):
        (WebCore::LinkLoader::linkLoadedTimerFired):
        (WebCore::LinkLoader::notifyFinished):
        (WebCore::LinkLoader::loadLink):
        * loader/LinkLoader.h: Added.
        * loader/LinkLoaderClient.h: Added.
        (WebCore::LinkLoaderClient::~LinkLoaderClient):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@89146 268f45cc-cd09-0410-ab3c-d52691b4dbfc