CrOS FilesApp: Crostini shared path management

New JS class Crostini to manage crostini shared paths.
Record paths that are shared, and do not show 'Share with Linux'
option for paths that are already shared.

Update Crostini UI test:
 * Verify that 'Share with Linux' is not shown for files (only dirs).
 * Fixed drive_welcome.css path.

Bug: 878324
Change-Id: I6b8445d0a127043f9d24126384267284af5ed7ab
Reviewed-by: Luciano Pacheco <>
Commit-Queue: Joel Hockey <>
Cr-Commit-Position: refs/heads/master@{#590211}
14 files changed