[Offline pages] Fixing crashes caused by access to offline pages marked for deletion

This patch fixes the problems caused by:
* HasOfflinePages
* GetPageByBookmarkId
* GetPageByOfflineURL
* GetPageByOnlineURL
All of which allow for access to the offline page that is marked for
deletion, which means there is no valid bookmark corresponding to them.
This leads to crashes in the bookmark UI, as well as exposing
functionality that should not work.

BUG=560424

Review URL: https://codereview.chromium.org/1475883003

Cr-Commit-Position: refs/heads/master@{#361746}
2 files changed