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Now they are defined the same. Use `is_linux` everywhere to avoid
confusion.

Bug: 1132395
Change-Id: Ib6288cefb2b3447b7ead54632f31dd34d778dd6c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2431777Reviewed-by: Ted Meyer <tmathmeyer@chromium.org>
Reviewed-by: John Rummell <jrummell@chromium.org>
Commit-Queue: Xiaohan Wang <xhwang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#810797}