This is related with crbug/770871, as the next step.

During P2P implementation, there should be:
2. Move files in the abandoned dir that are referenced by persistent pages to the new external directory, and update the persistent page entries.

Expectation is after the task, the abandoned dir will be only left the pages which are:
1. files for persistent pages failed during moving operation, or
2. left behind by mistake by other file operations (even for temporary pages, like the db entry was removed but the files are not removed.)