Lacros: Preparation for lacros C++ macro flip.

Lacros chrome is now built as target_os="chromeos" but C++ macros
OS_LINUX and OS_CHROMEOS has not been flipped. More on
https://crrev.com/c/2658161.

This ensures that lacros-chrome is consistent after such flip.

Bug: 1171629
Change-Id: I57490156357cca838435a7b893773035a6286eec
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2658704


Auto-Submit: Yuta Hijikata <ythjkt@chromium.org>
Commit-Queue: Dale Curtis <dalecurtis@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#848822}