Add details about load balancers and firewalls

Description:
This PR expands the explanation of the mobile browser web access process to cover how load balancers and firewalls are involved. This provides additional context on the infrastructure supporting connectivity.

Changes:

1. Added load balancer handling between browser and web servers
- When a DNS lookup is made to get Google's servers, it returns an IP address for a load balancer in front of the web servers
- The requests go to the load balancer, which it distributes across Google's many web servers to balance traffic

2. Added details on firewall roles
- There can be firewalls integrated into the mobile provider's infrastructure, which analyze traffic and block prohibited content
- Google's datacenters maintain tightly controlled firewalls to filter malicious requests while allowing valid traffic

These additions give better perspective on the intermediate infrastructure between a mobile browser and backend website servers. Let me know if any details should be modified!