For users that wish to disable local discovery in Chrome, a runtime flag is more convenient that the current combination of build flags (enable_local_discovery and enable_mdns).  This bug tracks the following tasks:

1. Add a flag, --enable_local_discovery that defaults true and guards running any local discovery service or UX, including Media Router related code (what is currently behind enable_local_discovery build flag).

2. Remove enable_local_discovery build flag.  Make sure that Chromium can still be built without printing support (some developers want to do this).

3. (Optional) Remove enable_mdns build flag unless the net team wants to keep it.  (There may be features other than local service discovery that use it, so we don't want to disable the code path entirely when --enable_local_discovery=0).