Add mutex lock for flagged request on Chromium

[younesaassila]

Current implementation lets all requests of certain type through the proxy while original request gets processed.
Should add some sort of mutex lock to hold all requests of type x when some request of type x gets flagged.

[younesaassila]

FIXME: If Full Mode times out, a DisableFullModeResponse message must be sent to the page/worker scripts in order to unlock the mutex for the request type.