Reviewed by David Levin.

        ThreadableLoaderClient needs willSendRequest method
        https://bugs.webkit.org/show_bug.cgi?id=54688

        No new tests. Exposes no new functionality

        * WebCore.gypi:
        * loader/DocumentThreadableLoader.cpp:
        (WebCore::DocumentThreadableLoader::willSendRequest):
        (WebCore::DocumentThreadableLoader::didReceiveData):
        (WebCore::DocumentThreadableLoader::didReceiveCachedMetadata):
        * loader/DocumentThreadableLoaderClient.h: Added.
        (WebCore::DocumentThreadableLoaderClient::isDocumentThreadableLoaderClient):
        (WebCore::DocumentThreadableLoaderClient::willSendRequest):
        * loader/ThreadableLoaderClient.h:
        (WebCore::ThreadableLoaderClient::isDocumentThreadableLoaderClient):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@79110 268f45cc-cd09-0410-ab3c-d52691b4dbfc