Prepend common plugin sandbox profile to specific per-plugin one.
<rdar://problem/13044349>

Reviewed by Alexey Proskuryakov.

* PluginProcess/mac/PluginProcessMac.mm:
(WebKit::initializeSandbox):



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@140865 268f45cc-cd09-0410-ab3c-d52691b4dbfc