This change adds an --enable-network-service command-line flag to run with
NetworkService enabled.
To test: Run `cefclient --enable-network-service --disable-extensions`. The
application should start, load a website and exit without crashing.
Network-related handlers (for cookies, schemes, requests, etc) and extensions
are not expected to work yet.
There should be no functional change when running without the NetworkService
enabled.
