Add local_discovery.enabled pref to control access to chrome://devices

The pref controls the local discovery UI (chrome://devices) which shows
discoverable devices near the user as well as cloud devices registered
to them. The page also allows for the adding of printers/devices, a
capability that administrators may want to restrict.

The pref defaults to true.

A future CL will add the policy configurations.

Bug: 1011582
Change-Id: I84a7ca5d22893b0a5cb0bf7948375ad8ef426288
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1975292
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Daniel Hosseinian <dhoss@chromium.org>
Cr-Commit-Position: refs/heads/master@{#726249}