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A previous CL made opening files on special volumes via the file picker
and file handling APIs work, this also fixes the entry point for
drag&drop in a similar manner.

The bulk of this change is changing NativeFileSystemMangerImpl to use
the file system backends instead of base::File to determine if a path
is a file or a directory.

Bug: 1132102
Change-Id: I6c64356ea285b717ce63afc9c34d097bcde08016
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2432808Reviewed-by: Victor Costan <pwnall@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Marijn Kruisselbrink <mek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#812944}