Most of these cases can be cleaned up by either renaming the function to end in ForTesting (which, like UNIT_TEST, we have a presubmit test for), or by moving code into a test support target (like ShadowingAtExitManager, which can probably just move into base/test/).

Ideally we could eliminate all such cases and have fewer ways that headers preprocess differently in different translation units.

Discussed in this thread: https://groups.google.com/a/chromium.org/d/topic/cxx/OUrPkZGaZCE/discussion