For <iframe src="some.swf" sandbox="...">, we should bail out of actually loading the plug-in data as soon as possible. Spun off from https://bugs.webkit.org/show_bug.cgi?id=39323. To resolve 39323, I planned on just making sure the plugin code itself is never instantiated which both prevents the described crash and prevents the security/sandboxing exploits of having the plugin code executing. But we should actually avoid actually loading anymore of the plugin data itself once we know it violates the sandboxing flags.