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Currently OS_LINUX is defined on Chrome OS build,
but it is being unset. This is the preparation to keep the
current behavior.

Bug: 1110266
Test: Built locally.
Change-Id: I773b4d46bc2e1f1cba9e3b8d0de5503d4e40eb92
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2449610
Commit-Queue: Hidehiko Abe <hidehiko@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Auto-Submit: Hidehiko Abe <hidehiko@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#813779}