Before this fix an iterator to `connections_` which requires a lock
would be dereferenced outside an unlock operation because the `it` taken
from the map isn't understood as guarded by the same lock.

This takes a Ref<NodeLink> before unlocking which'll keep the link
reference alive even if `connections_` is concurrently modified and the
entry removed (or replaced).

(cherry picked from commit 1f2cbf5833d7f00d3fcbfd1f3ef0c1aff10c04cd)

Bug: 1523704
Change-Id: I6f6fe4e34ec2c8268d4e7f33965a13e3b10f9f92
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5254025
Commit-Queue: Peter Boström <pbos@chromium.org>
Reviewed-by: Ken Rockot <rockot@google.com>
Commit-Queue: Ken Rockot <rockot@google.com>
Auto-Submit: Peter Boström <pbos@chromium.org>
Cr-Original-Commit-Position: refs/heads/main@{#1254709}
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5260163
Cr-Commit-Position: refs/branch-heads/6099@{#1916}
Cr-Branched-From: e6ee4500f7d6549a9ac1354f8d056da49ef406be-refs/heads/main@{#1217362}