FilesApp trash race fix

I just noticed another flake in
https://ci.chromium.org/p/chromium/builders/ci/linux-chromeos-rel/42616.

1/ The in-progress map is changed to store a globally unique identifier
so that multiple deletes across volumes cannot collide.

2/ The in-progress map was being cleared after the *.trashinfo files was
written, but before the file is moved. This is now changed so that we
only clear the map once the file is moved.

Bug: 953310
Change-Id: I49c9c75878288f2f1a0be4447c1885a07c873bc6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2521981
Auto-Submit: Joel Hockey <joelhockey@chromium.org>
Reviewed-by: Noel Gordon <noel@chromium.org>
Commit-Queue: Joel Hockey <joelhockey@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824750}