Change to default directory when removable entry list is unmounted.

Fixes bug where file list and breadcrumbs don't updated after removable
entry lists are unmounted.
Added unmount for removable test volumes, to test an unmount event.

Bug: 918795
Change-Id: I13ea71b6db9dff5ac2e59982eef7f6e34457fe31
Reviewed-on: https://chromium-review.googlesource.com/c/1465962
Commit-Queue: Alli Murray <alliemurray@google.com>
Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Cr-Commit-Position: refs/heads/master@{#632481}
9 files changed