[LNA] Link blink runtime feature and base feature flag

The permissions policy runtime feature doesn't do anything if the
underlying network::features::kLocalNetworkAccessChecks feature is not
enabled, so this updates it to be linked to the underlying feature flag
instead of being under "experimental". This means that for local testing
users will only have to enable
chrome://flags/#local-network-access-check to also make the permissions
policy available.

Change-Id: I5c65ee187e081021f2df7ce54083f30b2e2aed04
Bug: 400455013
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6520709
Reviewed-by: Hubert Chao <hchao@chromium.org>
Reviewed-by: Rick Byers <rbyers@chromium.org>
Commit-Queue: Chris Thompson <cthomp@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1458278}