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This is a first draft of a command source that handles basic
Chromium commands independent of actual content (like tabs, windows,
bookmarks, etc.)

The purpose for now is just to provide something that can be interacted
with when the view layer is added. Accordingly, commands have  been
chosen on the basis of: it's something we would like to implement
and it has a cross-platform translation that works for this context.

(NB: I didn't try particularly hard to find all of these)

There's also a stubbed out fuzzy finder which is not fuzzy in the least
at the moment.

Bug: 1014639
Change-Id: I08282aed82ba9fb1c62e592088e774a02479142e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2327820
Commit-Queue: Leonard Grey <lgrey@chromium.org>
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#793635}