Reviewed by Dan Bernstein, Mark Rowe and Kevin Decker.

        Add the plug-in host manager singleton.
        
        * Plugins/Hosted/NetscapePluginHostManager.h: Added.
        * Plugins/Hosted/NetscapePluginHostManager.mm: Added.
        
        (WebKit::NetscapePluginHostManager::hostForPackage):
        If there's an existing host proxy available, just return it. Otherwise spawn a new plug-in host and
        create a new plug-in host proxy from the new plug-in host port.
        
        (WebKit::NetscapePluginHostManager::spawnPluginHost):
        Pass the plug-in host path and the preferred CPU type to the plug-in agent. When the plug-in host has
        finished launching, pass it the path to the plug-in bundle.
        
        (WebKit::NetscapePluginHostManager::initializeVendorPort):
        Check in with the plug-in agent and get the new plug-in vendor port back.



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@38808 268f45cc-cd09-0410-ab3c-d52691b4dbfc