[presubmit] Enable runtime/threadsafe_fn linter checking.

[presubmit] Enable runtime/threadsafe_fn linter checking.

This enables linter checking for "runtime/threadsafe_fn" violations
during presubmit and instead marks the few known exceptions that we
allow explicitly.

R=jochen@chromium.org

Review URL: https://codereview.chromium.org/1369673003

Cr-Commit-Position: refs/heads/master@{#30933}