Chrome Version: 68.0.3413.0 (Developer Build) (64-bit).

Currently we check if the file exists on disk for all download items in download system. For each item, we post one file thread call to do blocking IO. So it may have hundreds of thread hoops if the user has many
download items in the history db.

offline_items_collection::FileExistenceChecker is a class used by offline
page team that only post one function to the file thread, and do all blocking IO together and reply to UI thread. This is cleaner.