Cleanup destruction of URLRequestContextGetter (#12305)

- Add Leak detector
 - Indicate shutdown of request context from Browser Context
 - Change stored references to URLRequestContextGetter to use BrowserContext
 - Destroy session properties explicitly

atom/browser/net/atom_url_request_job_factory.cc

@@ -92,6 +92,7 @@ void AtomURLRequestJobFactory::Clear() {
   for (auto& it : protocol_handler_map_)
     delete it.second;
   protocol_handler_map_.clear();
+  original_protocols_.clear();
 }
 
 net::URLRequestJob* AtomURLRequestJobFactory::MaybeCreateJobWithProtocolHandler(