Reviewed by Sam Weinig.

        <rdar://problem/6562220> 
        CrashTracer: [USER] 21 crashes in Safari at com.apple.WebKit • WebKit::NetscapePluginHostProxy::port
        
        Make the handling of crashes in the plug-in host more robust.
        
        * Plugins/Hosted/NetscapePluginHostProxy.h:
        Add m_portSet.
        
        * Plugins/Hosted/NetscapePluginHostProxy.mm:
        (WebKit::NetscapePluginHostProxy::NetscapePluginHostProxy):
        Initialize m_portSet.
        
        (WebKit::NetscapePluginHostProxy::~NetscapePluginHostProxy):
        Free m_portSet.
        
        (WebKit::NetscapePluginHostProxy::processRequests):
        Listen for messages on the port set. If we get a message to the port death notification port,
        then call pluginHostDied. Otherwise, process the message.
        
        * Plugins/Hosted/NetscapePluginInstanceProxy.h:
        * Plugins/Hosted/NetscapePluginInstanceProxy.mm:
        (WebKit::NetscapePluginInstanceProxy::cleanup):
        Factor code that should be shared between destroy() and pluginHostDied() into cleanup.
        
        (WebKit::NetscapePluginInstanceProxy::destroy):
        Call cleanup().
        
        (WebKit::NetscapePluginInstanceProxy::pluginHostDied):
        Call cleanup().
        
        (WebKit::NetscapePluginInstanceProxy::processRequestsAndWaitForReply):
        Call NetscapePluginHostProxy::processRequests.
        
        * Plugins/Hosted/ProxyInstance.mm:
        (WebKit::ProxyInstance::invalidate):
        Add a null check for the host proxy.



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@40733 268f45cc-cd09-0410-ab3c-d52691b4dbfc