Add 'chromeos' as one of the DEPS_OS_CHOICES.
Add 'chromeos' as one of the DEPS_OS_CHOICES.

This makes 'checkout_chromeos' conditions visible to '--deps=all'
processing.

R=ehmaldonado@google.com

Bug: 928829
Change-Id: I1fe3d73117203d549c7039f7621eecadd3ac344e
Reviewed-on: https://chromium-review.googlesource.com/c/1454849
Reviewed-by: Edward Lesmes <d05609f027dc667cc37492b1dc1bdea5d60fa82a@chromium.org>
Commit-Queue: Edward Lesmes <d05609f027dc667cc37492b1dc1bdea5d60fa82a@chromium.org>

CoherentLabs/depot_tools,CoherentLabs/depot_tools