Mojo: Write some tests for public/system/macros.h.

[Reland: Originally committed r252593, reverted r252596. We should revoke my
commit privileges.]

I really set out to also write no-compile tests, but they're currently totally
disabled. :(

R=sky@chromium.org

Review URL: https://codereview.chromium.org/175343002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252623 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252623