Adding new context-menu icons for crostini apps

New icons include:
 - Shutting down the vm from the terminal's context menu
 - Switching to {low,high}-density mode for a crostini apps

The icons were created by sgabriel@ and converted to .icon format using
a skiafy tool (http://evanstade.github.io/skiafy/).

Bug: 930858
Change-Id: I9b4b952c810685f5e067a63c9749ad853cc849f3
Reviewed-on: https://chromium-review.googlesource.com/c/1490358
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Evan Stade <estade@chromium.org>
Commit-Queue: Nic Hollingum <hollingum@google.com>
Cr-Commit-Position: refs/heads/master@{#636164}
6 files changed