Unreviewed trivial !ENABLE(NETSCAPE_PLUGIN_API) build fix

Add an intermediate pluginName string variable instead of
accessing plugin.info outside the feature #ifdef.

* UIProcess/WebPageProxy.cpp:
(WebKit::WebPageProxy::unavailablePluginButtonClicked):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@144015 268f45cc-cd09-0410-ab3c-d52691b4dbfc