Enforce no use of windows.h in atomicops.h

atomicops.h was fixed in January to not depend on Windows.h anymore.
Update the comment to reflect that, and enforce it in the unit test.

Bug: 559247, 796644
Change-Id: I95762b818d135f6f400e1b9aa66567464aef8951
Reviewed-on: https://chromium-review.googlesource.com/956668Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542240}

base/atomicops.h

@@ -145,8 +145,8 @@ Atomic64 Release_Load(volatile const Atomic64* ptr);
 }  // namespace base
 
 #if defined(OS_WIN)
-// TODO(jfb): The MSVC header includes windows.h, which other files end up
-//            relying on. Fix this as part of crbug.com/559247.
+// TODO(jfb): Try to use base/atomicops_internals_portable.h everywhere.
+// https://crbug.com/559247.
 #  include "base/atomicops_internals_x86_msvc.h"
 #else
 #  include "base/atomicops_internals_portable.h"

base/win/win_includes_unittest.cc

@@ -5,6 +5,7 @@
 // This file ensures that these header files don't include Windows.h and can
 // compile without including Windows.h. This helps to improve compile times.
 
+#include "base/atomicops.h"
 #include "base/files/file_util.h"
 #include "base/files/platform_file.h"
 #include "base/process/process_handle.h"