this isn't urgent, so moving to M-71

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/c74530e0681009347cfa2525049fbbe07f84e26b

commit c74530e0681009347cfa2525049fbbe07f84e26b
Author: Luciano Pacheco <lucmult@chromium.org>
Date: Fri Oct 05 00:58:13 2018

Remove directory tree menu item

This is dead code, since "Add service" migrated to "gear menu"
crrev.com/c/1139944.

Remove:
1. ADD_NEW_SERVICES_MENU constant from VolumeManagerCommon.RootType.
2. Style and icons specific for displaying "Add service" in the
directory tree.
3. Icons that was used for "Add service" in directory tree.
4. NavigationModelMenuItem and MenuItem types.

NOTE: Translation strings are still in use in the gear menu so we won't
remove them.

Bug:  869252 
Change-Id: Ic6c0abf74dff6cf2ebf2821b88d930dcd1653bbf
Reviewed-on: https://chromium-review.googlesource.com/c/1260882
Reviewed-by: Joel Hockey <joelhockey@chromium.org>
Commit-Queue: Luciano Pacheco <lucmult@chromium.org>
Cr-Commit-Position: refs/heads/master@{#596950}
[modify] https://crrev.com/c74530e0681009347cfa2525049fbbe07f84e26b/ui/file_manager/file_manager/common/js/volume_manager_common.js
[modify] https://crrev.com/c74530e0681009347cfa2525049fbbe07f84e26b/ui/file_manager/file_manager/foreground/css/file_types.css
[delete] https://crrev.com/bc6c28fcebf4e88f671fba15cfbb9e891887cc6a/ui/file_manager/file_manager/foreground/images/volumes/2x/add.png
[delete] https://crrev.com/bc6c28fcebf4e88f671fba15cfbb9e891887cc6a/ui/file_manager/file_manager/foreground/images/volumes/add.png
[modify] https://crrev.com/c74530e0681009347cfa2525049fbbe07f84e26b/ui/file_manager/file_manager/foreground/js/file_manager.js
[modify] https://crrev.com/c74530e0681009347cfa2525049fbbe07f84e26b/ui/file_manager/file_manager/foreground/js/navigation_list_model.js
[modify] https://crrev.com/c74530e0681009347cfa2525049fbbe07f84e26b/ui/file_manager/file_manager/foreground/js/navigation_list_model_unittest.js
[modify] https://crrev.com/c74530e0681009347cfa2525049fbbe07f84e26b/ui/file_manager/file_manager/foreground/js/ui/directory_tree.js

No change in behaviour expected from this change, this is just a code cleanup.