The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/2952ef5191550d9b246f4b62a442b5cff4411036

commit 2952ef5191550d9b246f4b62a442b5cff4411036
Author: Yafei Duan <romax@chromium.org>
Date: Wed Nov 15 21:09:17 2017

[Offline Pages] Add missing sql connection pointer checks.

There were some tasks missing raw pointer check for sql connection.
Fixing by adding checks and return errors.

Bug:  772206 
Change-Id: I31c997974cca644a61db54eba645ca988dca5d6d
Reviewed-on: https://chromium-review.googlesource.com/762044
Reviewed-by: Filip Gorski <fgorski@chromium.org>
Commit-Queue: Yafei Duan <romax@chromium.org>
Cr-Commit-Position: refs/heads/master@{#516828}
[modify] https://crrev.com/2952ef5191550d9b246f4b62a442b5cff4411036/components/offline_pages/core/model/clear_digest_task.cc
[modify] https://crrev.com/2952ef5191550d9b246f4b62a442b5cff4411036/components/offline_pages/core/model/clear_storage_task.cc
[modify] https://crrev.com/2952ef5191550d9b246f4b62a442b5cff4411036/components/offline_pages/core/model/mark_page_accessed_task.cc

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/b3052a4adbc8b8a73b8a31447101da2be04e9cca

commit b3052a4adbc8b8a73b8a31447101da2be04e9cca
Author: Yafei Duan <romax@chromium.org>
Date: Tue Nov 28 23:13:14 2017

[Offline Pages] DCHECK raw pointers for tasks and null callbacks.

Most tasks are taking raw pointers and they needs to be checked against
nullptr. This CL fixed the potential issue.

Bug:  772206 
Change-Id: Ifab61fd75ed88c9d03fd0af8088454148c88144a
Reviewed-on: https://chromium-review.googlesource.com/792261
Reviewed-by: Peter Williamson <petewil@chromium.org>
Commit-Queue: Yafei Duan <romax@chromium.org>
Cr-Commit-Position: refs/heads/master@{#519890}
[modify] https://crrev.com/b3052a4adbc8b8a73b8a31447101da2be04e9cca/components/offline_pages/core/model/add_page_task.cc
[modify] https://crrev.com/b3052a4adbc8b8a73b8a31447101da2be04e9cca/components/offline_pages/core/model/clear_digest_task.cc
[modify] https://crrev.com/b3052a4adbc8b8a73b8a31447101da2be04e9cca/components/offline_pages/core/model/clear_legacy_temporary_pages_task.cc
[modify] https://crrev.com/b3052a4adbc8b8a73b8a31447101da2be04e9cca/components/offline_pages/core/model/clear_storage_task.cc
[modify] https://crrev.com/b3052a4adbc8b8a73b8a31447101da2be04e9cca/components/offline_pages/core/model/create_archive_task.cc
[modify] https://crrev.com/b3052a4adbc8b8a73b8a31447101da2be04e9cca/components/offline_pages/core/model/delete_page_task.cc
[modify] https://crrev.com/b3052a4adbc8b8a73b8a31447101da2be04e9cca/components/offline_pages/core/model/mark_page_accessed_task.cc
[modify] https://crrev.com/b3052a4adbc8b8a73b8a31447101da2be04e9cca/components/offline_pages/core/model/persistent_pages_consistency_check_task.cc
[modify] https://crrev.com/b3052a4adbc8b8a73b8a31447101da2be04e9cca/components/offline_pages/core/model/temporary_pages_consistency_check_task.cc