Plug-ins should be able to update the mouse cursor

Plug-ins should be able to update the mouse cursor
https://bugs.webkit.org/show_bug.cgi?id=50399

Reviewed by Sam Weinig.

WebKit2:

Call WKEnableSettingCursorWhenInBackground.

* PluginProcess/mac/PluginProcessMainMac.mm:
(WebKit::PluginProcessMain):

WebKitLibraries:

Add WKEnableSettingCursorWhenInBackground.

* WebKitSystemInterface.h:
* libWebKitSystemInterfaceLeopard.a:
* libWebKitSystemInterfaceSnowLeopard.a:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@73159 268f45cc-cd09-0410-ab3c-d52691b4dbfc