Add context menu for removable root entry list.

- Context menu should appear when right-click on root label.
- Command to unmount should be visible in context menu for removable roots and removable USBs.
- Partitions in the directory tree should not have an unmount in the context menu.

Bug: 918795
Change-Id: I025b1bb7a7cd15ce9610909e98b514f6ccab74e1
Reviewed-on: https://chromium-review.googlesource.com/c/1460201
Commit-Queue: Alli Murray <alliemurray@google.com>
Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Cr-Commit-Position: refs/heads/master@{#632107}
3 files changed