build: Fix OS buildflag comment

BUILDFLAG(IS_CHROMEOS) is also supported. Fix the comment in
build_config.h to reflect that.

Bug: 12340436
Change-Id: I2d5cfd236a2e3a7121e9f7e4223f08c84c5f1e7b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3415176


Commit-Queue: Xiaohan Wang <xhwang@chromium.org>
Auto-Submit: Xiaohan Wang <xhwang@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/main@{#963063}