Reviewed by Darin Adler.

        WebKit r30323 added -_webcore_MIMEType to fix issues with
        incorrect MIME types in NS[HTTP]URLResponse objects.  However,
        uses of -[NSURLResponse MIMEType] still persist in WebKit that
        should be switched to use -_webcore_MIMEType.  Note that
        -[WebDataSource _responseMIMEType] calls back into WebCore to
        get the MIME type from the ResourceResponse object, which has
        already retrieved it via -_webcore_MIMEType.

        * Plugins/Hosted/HostedNetscapePluginStream.mm:
        (WebKit::HostedNetscapePluginStream::didReceiveResponse): Use
        -_webcore_MIMEType.
        * Plugins/WebBaseNetscapePluginStream.mm:
        (WebNetscapePluginStream::didReceiveResponse): Ditto.
        * Plugins/WebNetscapePluginView.mm:
        (-[WebNetscapePluginView pluginView:receivedData:]): Ditto.
        * Plugins/WebPluginController.mm:
        (-[WebPluginController pluginView:receivedResponse:]): Ditto.
        * WebView/WebHTMLRepresentation.mm:
        (-[WebHTMLRepresentation _isDisplayingWebArchive]): Use
        -[WebDataSource _responseMIMEType] instead.
        * WebView/WebPDFRepresentation.m:
        (-[WebPDFRepresentation finishedLoadingWithDataSource:]): Ditto.
        * WebView/WebPDFView.mm:
        (-[WebPDFView menuForEvent:]): Ditto.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@41783 268f45cc-cd09-0410-ab3c-d52691b4dbfc