So the goal of this is blocked by some concerns.
I'll just keep the way to use separate directories based on LifetimePolicy::LifetimeType for now. Even though they're using the same app data folder.
Patch kinda ready at
https://codereview.chromium.org/1928393003

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/84146c7d410b92010d8599b7fea3f9de345dfdbc

commit 84146c7d410b92010d8599b7fea3f9de345dfdbc
Author: romax <romax@chromium.org>
Date: Thu May 12 23:34:53 2016

[Offline Pages] Adding more expiration logic.

BUG= 606088 

Review-Url: https://codereview.chromium.org/1970953002
Cr-Commit-Position: refs/heads/master@{#393397}

[modify] https://crrev.com/84146c7d410b92010d8599b7fea3f9de345dfdbc/components/offline_pages/offline_page_storage_manager.cc
[modify] https://crrev.com/84146c7d410b92010d8599b7fea3f9de345dfdbc/components/offline_pages/offline_page_storage_manager.h
[modify] https://crrev.com/84146c7d410b92010d8599b7fea3f9de345dfdbc/components/offline_pages/offline_page_storage_manager_unittest.cc