Chrome OS version: 67.0.3396.99 (OFFICIAL)

Steps to reproduce:
1. Mark one or more Drive files as "Available offline" in the Files app. Choose files that are not virtual (Google docs etc) files. 
2. Go to the "Offline" folder under Google Drive

Expected result:
All files that had been marked "Available offline" are visible in the "Offline" folder.

Observed result:
Only virtual files are visible in the "Offline" folder. 

We should either 
1. Show all "Available offline" files in the "Offline" folder.
or
2. Remove the option to mark non-virtual files as "Available offline".