GitLab

Currently code dchecks on the existence of the offline page for a given
bookmark, which might not be the case.
Removing DCHECK and only continuing execution if the offline page exists.

BUG=560518
R=jianli@chromium.org

Review URL: https://codereview.chromium.org/1473043002

Cr-Commit-Position: refs/heads/master@{#361363}