Engage the presubmit warning for OS_MACOSX

OS_MACOSX will still work for code already in the tree, but warn
to prevent new instances from landing.

Bug: 1105907
Change-Id: Ia811e352ab7ec688bad5e8e7b31868777dd75c10
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2328330
Commit-Queue: Avi Drissman <avi@chromium.org>
Commit-Queue: Dirk Pranke <dpranke@google.com>
Auto-Submit: Avi Drissman <avi@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@google.com>
Cr-Commit-Position: refs/heads/master@{#793021}

PRESUBMIT.py

@@ -1306,7 +1306,6 @@ _VALID_OS_MACROS = (
     'OS_IOS',
     'OS_LINUX',
     'OS_MAC',
-    'OS_MACOSX',
     'OS_NACL',
     'OS_NACL_NONSFI',
     'OS_NACL_SFI',

PRESUBMIT_test.py

@@ -275,6 +275,7 @@ class InvalidOSMacroNamesTest(unittest.TestCase):
     lines = ['#if defined(OS_WINDOWS)',
              ' #elif defined(OS_WINDOW)',
              ' # if defined(OS_MAC) || defined(OS_CHROME)',
+             '# else  // defined(OS_MACOSX)',
              '#endif  // defined(OS_MACOS)']
     errors = PRESUBMIT._CheckForInvalidOSMacrosInFile(
         MockInputApi(), MockFile('some/path/foo_platform.cc', lines))