Also, as this is hard to track down, always DCHECK position after ReadBlock().

Change-Id: Ie32c3a311dd8df91f651b6d82ccacc7c95e6fde0
Reviewed-on: https://chromium-review.googlesource.com/528196
Commit-Queue: Marja Hölttä <marja@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Reviewed-by: Daniel Vogelheim <vogelheim@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45811}