One change to make sure we got included is the UMA for offline page storage in use
https://chromium-review.googlesource.com/c/chromium/src/+/710094
and subsequent changes:
https://chromium-review.googlesource.com/c/chromium/src/+/722033
and
https://chromium-review.googlesource.com/c/chromium/src/+/737957

The OfflinePages.ClearStoragePreRunUsage2.* series of histograms must be migrated too. Their most recent work is being tracked in  issue 775321 .

They are reported from offline_page_storage_manager.cc it has tests in offline_page_storage_manager_unittest.cc that should be ported along.

Go over commit history of OfflinePageModelImpl* before closing this bug.

The metric logged in OfflinePageStorageManager should be migrated to the new ClearStorageTask, with some modification to the DB query part so that all pages can be fetched and considered while reporting the UMA.

Other than that, there's nothing else that needs to be migrated from OPMImpl to OPMTaskified.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/cfa1dc4c39595b19b71a57e6c0d9c1bdf12c6ee9

commit cfa1dc4c39595b19b71a57e6c0d9c1bdf12c6ee9
Author: Carlos Knippschild <carlosk@chromium.org>
Date: Thu Feb 08 22:32:23 2018

Re-add metric to report the total count of offline.

This CL introduces a new metric to replace OfflinePages.SavedPageCount
which was not carried over into the new taskified Offline Pages metadata
model. A new one was created because I am unsure if the reporting
characteristics of this metric are maintained between the old and new
metadata model.

Bug:  778813 
Change-Id: If31b8e71b6ab487ff732fa72f99ba3755c38e3b1
Reviewed-on: https://chromium-review.googlesource.com/905567
Commit-Queue: Carlos Knippschild <carlosk@chromium.org>
Reviewed-by: Dmitry Titov <dimich@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#535548}
[modify] https://crrev.com/cfa1dc4c39595b19b71a57e6c0d9c1bdf12c6ee9/components/offline_pages/core/model/offline_page_model_taskified.cc
[modify] https://crrev.com/cfa1dc4c39595b19b71a57e6c0d9c1bdf12c6ee9/components/offline_pages/core/model/offline_page_model_taskified_unittest.cc
[modify] https://crrev.com/cfa1dc4c39595b19b71a57e6c0d9c1bdf12c6ee9/components/offline_pages/core/offline_page_types.h
[modify] https://crrev.com/cfa1dc4c39595b19b71a57e6c0d9c1bdf12c6ee9/tools/metrics/histograms/histograms.xml

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/864f13d0ead9799b19bd3d472443aac81f98f2fc

commit 864f13d0ead9799b19bd3d472443aac81f98f2fc
Author: Dan Harrington <harringtond@chromium.org>
Date: Fri Mar 02 23:47:53 2018

Remove the unused OfflinePageTestStore

Change-Id: I0ee9ca1b2bd652c4a2de9fd65ba9ee2eb8d1da1c
Bug:  778813 
Reviewed-on: https://chromium-review.googlesource.com/946449
Commit-Queue: Dan H <harringtond@chromium.org>
Reviewed-by: Carlos Knippschild <carlosk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#540681}
[modify] https://crrev.com/864f13d0ead9799b19bd3d472443aac81f98f2fc/chrome/browser/offline_pages/offline_page_utils_unittest.cc
[modify] https://crrev.com/864f13d0ead9799b19bd3d472443aac81f98f2fc/components/offline_pages/core/BUILD.gn
[modify] https://crrev.com/864f13d0ead9799b19bd3d472443aac81f98f2fc/components/offline_pages/core/model/offline_page_model_taskified_unittest.cc
[delete] https://crrev.com/48d93ebdf2a13e147aa9d2bda3797725592099aa/components/offline_pages/core/offline_page_test_store.cc
[delete] https://crrev.com/48d93ebdf2a13e147aa9d2bda3797725592099aa/components/offline_pages/core/offline_page_test_store.h

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/28b6c87f79900c1de4105c0a594337b3f1f77031

commit 28b6c87f79900c1de4105c0a594337b3f1f77031
Author: Dan Harrington <harringtond@chromium.org>
Date: Wed Mar 07 22:55:46 2018

Remove OfflinePageMetadataStore

OfflinePageMetadataStore was only referenced in the unittest, now it has been
removed. A few methods/types were migrated to OfflinePageMetadataStoreSQL,
others were removed because the were no longer in use.

I removed some tests that only tested the removed methods. Other methods
that were used to test the database and were moved to the test file, like
GetOfflinePages and AddOfflinePage.

offline_page_metadata_store_unittest.cc will be renamed
offline_page_metadata_store_sql_unittest.cc in a follow-up.

Bug:  778813 
Change-Id: I6315cd5fb43b8a56b84dcede8c388286dd96e39c
Reviewed-on: https://chromium-review.googlesource.com/949502
Reviewed-by: Yafei Duan <romax@chromium.org>
Reviewed-by: Carlos Knippschild <carlosk@chromium.org>
Commit-Queue: Dan H <harringtond@chromium.org>
Cr-Commit-Position: refs/heads/master@{#541621}
[modify] https://crrev.com/28b6c87f79900c1de4105c0a594337b3f1f77031/components/offline_pages/core/BUILD.gn
[modify] https://crrev.com/28b6c87f79900c1de4105c0a594337b3f1f77031/components/offline_pages/core/model/delete_page_task.h
[modify] https://crrev.com/28b6c87f79900c1de4105c0a594337b3f1f77031/components/offline_pages/core/model/offline_page_model_taskified.cc
[delete] https://crrev.com/f4167f1f1ca664e11e81cbd7ec6a5dad5ed4f812/components/offline_pages/core/offline_page_metadata_store.h
[modify] https://crrev.com/28b6c87f79900c1de4105c0a594337b3f1f77031/components/offline_pages/core/offline_page_metadata_store_sql.cc
[modify] https://crrev.com/28b6c87f79900c1de4105c0a594337b3f1f77031/components/offline_pages/core/offline_page_metadata_store_sql.h
[modify] https://crrev.com/28b6c87f79900c1de4105c0a594337b3f1f77031/components/offline_pages/core/offline_page_metadata_store_unittest.cc

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/ec1efc85b38024c0a1a466d09f2c6d730d374648

commit ec1efc85b38024c0a1a466d09f2c6d730d374648
Author: Yafei Duan <romax@chromium.org>
Date: Thu Mar 22 02:15:39 2018

[Offline Pages] Merging maintenance tasks during startup.

This CL contains the following changes:
- Combined consistency checks for temporary/persistent pages, along with
  clearing legacy archives directory into the StartupMaintenanceTask.
  Also removed old files.
- Combined related tests.
- Adding a step to report storage usage for ClearStoragePreRunUsage2 at
  startup.
- Adding an ArchiveManager instance and a method AddPageWithoutDBEntry()
  to ModelTaskTestBase.

Bug:  778813 
Change-Id: I9af446da29dbbf912ae0a4f2d73531c224e81f80
Reviewed-on: https://chromium-review.googlesource.com/965634
Commit-Queue: Yafei Duan <romax@chromium.org>
Reviewed-by: Carlos Knippschild <carlosk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#544950}
[modify] https://crrev.com/ec1efc85b38024c0a1a466d09f2c6d730d374648/components/offline_pages/core/BUILD.gn
[delete] https://crrev.com/f9bbbe1a486850571e8ef8a8e0e3dd232e7632fa/components/offline_pages/core/model/clear_legacy_temporary_pages_task.cc
[delete] https://crrev.com/f9bbbe1a486850571e8ef8a8e0e3dd232e7632fa/components/offline_pages/core/model/clear_legacy_temporary_pages_task.h
[delete] https://crrev.com/f9bbbe1a486850571e8ef8a8e0e3dd232e7632fa/components/offline_pages/core/model/clear_legacy_temporary_pages_task_unittest.cc
[modify] https://crrev.com/ec1efc85b38024c0a1a466d09f2c6d730d374648/components/offline_pages/core/model/model_task_test_base.cc
[modify] https://crrev.com/ec1efc85b38024c0a1a466d09f2c6d730d374648/components/offline_pages/core/model/model_task_test_base.h
[modify] https://crrev.com/ec1efc85b38024c0a1a466d09f2c6d730d374648/components/offline_pages/core/model/offline_page_model_taskified.cc
[delete] https://crrev.com/f9bbbe1a486850571e8ef8a8e0e3dd232e7632fa/components/offline_pages/core/model/persistent_pages_consistency_check_task.cc
[delete] https://crrev.com/f9bbbe1a486850571e8ef8a8e0e3dd232e7632fa/components/offline_pages/core/model/persistent_pages_consistency_check_task.h
[delete] https://crrev.com/f9bbbe1a486850571e8ef8a8e0e3dd232e7632fa/components/offline_pages/core/model/persistent_pages_consistency_check_task_unittest.cc
[add] https://crrev.com/ec1efc85b38024c0a1a466d09f2c6d730d374648/components/offline_pages/core/model/startup_maintenance_task.cc
[add] https://crrev.com/ec1efc85b38024c0a1a466d09f2c6d730d374648/components/offline_pages/core/model/startup_maintenance_task.h
[add] https://crrev.com/ec1efc85b38024c0a1a466d09f2c6d730d374648/components/offline_pages/core/model/startup_maintenance_task_unittest.cc
[delete] https://crrev.com/f9bbbe1a486850571e8ef8a8e0e3dd232e7632fa/components/offline_pages/core/model/temporary_pages_consistency_check_task.cc
[delete] https://crrev.com/f9bbbe1a486850571e8ef8a8e0e3dd232e7632fa/components/offline_pages/core/model/temporary_pages_consistency_check_task.h
[delete] https://crrev.com/f9bbbe1a486850571e8ef8a8e0e3dd232e7632fa/components/offline_pages/core/model/temporary_pages_consistency_check_task_unittest.cc

The issue mentioned in #3 and #6 (ClearStoragePreRunUsage2 migration) is completed in the CL logged in #10. So closing this tracking bug.