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We have not shipped Windows Ash for years. There is a 1:1 mapping
between "use_ash" and "chromeos", so consolidate them.

TBR=oshima@chromium.org for theme_resources.grd

Bug: 673826
Test: compiles, bots
Change-Id: If76122c1089cde2ec2b7d8822a4ca0adde25e5f3
Reviewed-on: https://chromium-review.googlesource.com/699663Reviewed-by: James Cook <jamescook@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#506508}