[Plugins] delete ws_info regardless of window mode.

[Plugins] delete ws_info regardless of window mode.
https://bugs.webkit.org/show_bug.cgi?id=83004

Patch by Bang Kwang min <justine.bang@samsung.com> on 2012-04-17
Reviewed by Simon Hausmann.

m_npWindow.ws_info should be deleted even if plugin run as windowless mode to remove memory leak.
Every plugin works well after this patch without any crash.
Additionally, change casting syntax to C++ style.

* plugins/PluginView.cpp:
(WebCore::PluginView::stop):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@114358 268f45cc-cd09-0410-ab3c-d52691b4dbfc