Add 'Shut Down' option to context menus for Plugin VM

This CL adds a 'Shut Down' option to the shelf and launcher context
menus for the Plugin VM app. These will only show while the app is
open. For now we simply dispatch a StopVm message, but we may want to
expand this to also close the UI window once the shut down is complete.

Bug: 940319
Change-Id: I3acbbaf540384fad1dbb68e6226420b47d586793
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1621987
Commit-Queue: Timothy Loh <timloh@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#662512}
12 files changed