Change #ifndef OS_FOO to #if !defined(OS_FOO).

Consistently write it the same way as most of the code base. Then
include build/build_config.h as required to guarantee OS_FOO is properly
defined.

Change-Id: I30c9e5c96808469b8e6b6a92ecfaa9239e63568c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2277742
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Katie Dektar <katie@chromium.org>
Reviewed-by: Khushal <khushalsagar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#786509}