cert_provisioning: Fix invalidation handling

The invalidation topics in certificate provisioning are public, not
private.
Also, fix CertProvisioningWorker accidentally invalidating the weak
pointer it used to bind the OnShouldContinue callback.
Add a unit test for that case (fails without the fix).

Bug: 1146410
Change-Id: Ib3024d977de3de2a2b75de0c0b279dd0c8764883
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2523208


Commit-Queue: Pavol Marko <pmarko@chromium.org>
Reviewed-by: Michael Ershov <miersh@google.com>
Cr-Commit-Position: refs/heads/master@{#824935}