Bug 684708 - Allow setting forceAllowThirdPartyCookie on requests

In the nsIHttpChannelInternal interface there's a flag called forceAllowThirdPartyCookie that was introduced as a fix for the Bug 437174, allowing you to force sending 3rd party cookies if the user had disabled that from the preferences.
The reason behind adding such flag was that some addons need to be able to send those cookies (like a gmail/facebook scanner, etc.) and as they are installed and given permission by the user they are not untrusted parties. 

The only way to set that flag is on the XHR object after it had been opened, so there's no way to do it now from the main.js.