WebKit clients should be able to override loading of blocked plug-ins
https://bugs.webkit.org/show_bug.cgi?id=108968
<rdar://problem/13154516>

Reviewed by Sam Weinig.

Replace the shouldInstantiatePlugin callback with a new pluginLoadPolicy which is called regardless
of whether the plug-in is blocked or not. This lets clients override the plug-in load policy and
force loading of blacklisted plug-ins (and vice versa).

* UIProcess/API/C/WKPage.h:
* UIProcess/WebPageProxy.cpp:
(WebKit::WebPageProxy::getPluginPath):
* UIProcess/WebUIClient.cpp:
(WebKit::toWKPluginLoadPolicy):
(WebKit):
(WebKit::toPluginModuleLoadPolicy):
(WebKit::WebUIClient::pluginLoadPolicy):
* UIProcess/WebUIClient.h:
(WebUIClient):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@141923 268f45cc-cd09-0410-ab3c-d52691b4dbfc