Add check against NPObject accesses after plugin NP_Shutdown().

TestNetscapePlugIn now creates a copy of the plugin script
object class on the heap, and overwrites it with garbage in
NP_Shutdown, to trap any attempts to call into plugin-side NPObject code after the plugin DLL might have been unloaded, independent of OS-specific DLL unload behaviour. 

BUG=152006

Review URL: https://chromiumcodereview.appspot.com/19067006

git-svn-id: svn://svn.chromium.org/blink/trunk@154230 bbb929c8-8fbe-4397-9dbb-9b2b20218538