build: Define OS build flags in build_config.h

As discussed with build/ owners, we decide to directly define OS build
flags in build_config.h, instead of via a `buildflag_header` target in a
GN file, which is typically how build flags are defined. This helps
avoid the build speed impact caused by adding a new dependency on a GN
target in many many files, and avoid the developer overhead of adding
a new include and GN deps when using these build flags.

This also makes the migration much simpler. See go/os-buildflags-lsc.

In the future we may revisit this decision. Even if we change the plan
the current CL is still a legit intermediate step to do.

Bug: 1234043
Change-Id: I356831b7ab7c123fbe2caebbb80581420f59e058
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3366223
Reviewed-by: Nico Weber <thakis@chromium.org>
Owners-Override: Xiaohan Wang <xhwang@chromium.org>
Commit-Queue: Xiaohan Wang <xhwang@chromium.org>
Cr-Commit-Position: refs/heads/main@{#956626}

diff --git a/ipc/ipc_send_fds_test.cc b/ipc/ipc_send_fds_test.cc
index 0523d9b..dad1710 100644
--- a/ipc/ipc_send_fds_test.cc
+++ b/ipc/ipc_send_fds_test.cc

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-#include "build/os_buildflags.h"
+#include "build/build_config.h"
 
 #if BUILDFLAG(IS_MAC)
 extern "C" {