Reviewed by Brady Eidson.

        <rdar://problem/6660037> CrashTracer: [USER] 46 crashes in Safari at com.apple.WebCore • WebCore::CachedCSSStyleSheet::addClient + 53
        
        When revalidating a resource, calling addClient() on one client might cause another to get removed.
        
        - made CachedResource::addClient() non-virtual and added virtual didAddClient()
        - in CachedResource::switchClientsToRevalidatedResource() add all clients to the client set of the revalidated resource first
        - check if the client is still in the set before invoking didAddClient() for it
        
        No test case, I didn't manage to construct one. You need some combination of 304 revalidation, stylesheets that
        reference each other via @imports and reloading.

        * WebCore.base.exp:
        * loader/CachedCSSStyleSheet.cpp:
        (WebCore::CachedCSSStyleSheet::didAddClient):
        * loader/CachedCSSStyleSheet.h:
        * loader/CachedFont.cpp:
        (WebCore::CachedFont::didAddClient):
        * loader/CachedFont.h:
        * loader/CachedImage.cpp:
        (WebCore::CachedImage::didAddClient):
        * loader/CachedImage.h:
        * loader/CachedResource.cpp:
        (WebCore::CachedResource::addClient):
        (WebCore::CachedResource::addClientToSet):
        (WebCore::CachedResource::switchClientsToRevalidatedResource):
        * loader/CachedResource.h:
        * loader/CachedScript.cpp:
        (WebCore::CachedScript::didAddClient):
        * loader/CachedScript.h:
        * loader/CachedXSLStyleSheet.cpp:
        (WebCore::CachedXSLStyleSheet::didAddClient):
        * loader/CachedXSLStyleSheet.h:



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@44749 268f45cc-cd09-0410-ab3c-d52691b4dbfc