Seems like we're not going to hook with importance, instead we're targeting to move the cache pages to cache directory, which will be deleted in Android Settings->Apps->Chrome->Clear Cache.

Is there a design doc or other place I can read about the plan?

Answered my own question. go/separating-storage-for-offline-pages

There are still many issues to be considered after we move the OPC into cache directory, for example the consistency check since the 'clear cache' in system setting is not going through OfflinePageModel.

The known issues should be listed either in this issue or in the doc. If they don't refer to restricted information then they should preferably be listed here as the doc has restricted access.

Another issue might be after system clearing cache, since the folder is gone, we may get errors when trying to save the pages.
So consistency check and ensure the archive folder has created should be bundled somehow.

The reported problem from issue 725122 made me remember that one big requirement for this change is that it cannot have unexpected consequences over existing downloads. Just noting it here to make sure we keep this in mind.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/429de7772b63be050a48c7ba77b6a484c8e6f20e

commit 429de7772b63be050a48c7ba77b6a484c8e6f20e
Author: Yafei Duan <romax@chromium.org>
Date: Tue Oct 03 23:23:15 2017

[Offline Pages] Use cache directory for temporary pages.

For temporary offline pages, there is a new archive directory in app's
cache directory, which makes it possible to delete all temporary pages
when the user clicks 'Clear Cache' in Android settings.
The patch added a new file path, changed related behaviors in archive
manager when getting storage stats or all archive directories.
If during the creation of OfflinePageModel, the PathService::Get() fails
to get the temporary archive directory, there will be no temporary pages
saved for the Chrome Lifecycle, since the service couldn't get such a
path. So all attempts to save temporary pages will return failure.

Bug:  716141 
Change-Id: I4f7fc5cdd4907e2b1e90b5e40ad2358753c53efb
Reviewed-on: https://chromium-review.googlesource.com/667750
Commit-Queue: Yafei Duan <romax@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Reviewed-by: Filip Gorski <fgorski@chromium.org>
Reviewed-by: Justin DeWitt <dewittj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#506227}
[modify] https://crrev.com/429de7772b63be050a48c7ba77b6a484c8e6f20e/chrome/browser/offline_pages/android/offline_page_model_factory.cc
[modify] https://crrev.com/429de7772b63be050a48c7ba77b6a484c8e6f20e/chrome/browser/offline_pages/test_offline_page_model_builder.cc
[modify] https://crrev.com/429de7772b63be050a48c7ba77b6a484c8e6f20e/components/offline_pages/core/archive_manager.cc
[modify] https://crrev.com/429de7772b63be050a48c7ba77b6a484c8e6f20e/components/offline_pages/core/archive_manager.h
[modify] https://crrev.com/429de7772b63be050a48c7ba77b6a484c8e6f20e/components/offline_pages/core/archive_manager_unittest.cc
[modify] https://crrev.com/429de7772b63be050a48c7ba77b6a484c8e6f20e/components/offline_pages/core/model/clear_storage_task.cc
[modify] https://crrev.com/429de7772b63be050a48c7ba77b6a484c8e6f20e/components/offline_pages/core/offline_page_model_impl.cc
[modify] https://crrev.com/429de7772b63be050a48c7ba77b6a484c8e6f20e/components/offline_pages/core/offline_page_model_impl.h
[modify] https://crrev.com/429de7772b63be050a48c7ba77b6a484c8e6f20e/components/offline_pages/core/offline_page_model_impl_unittest.cc
[modify] https://crrev.com/429de7772b63be050a48c7ba77b6a484c8e6f20e/components/offline_pages/core/offline_page_storage_manager.cc
[modify] https://crrev.com/429de7772b63be050a48c7ba77b6a484c8e6f20e/components/offline_pages/core/offline_page_storage_manager_unittest.cc
[modify] https://crrev.com/429de7772b63be050a48c7ba77b6a484c8e6f20e/tools/metrics/histograms/histograms.xml