Introduce OS_NACL_SFI and OS_NACL_NONSFI macros.

PNaCl toolchain starts to support "Non-SFI build binary" mode.
Some sources are built both for SFI binary and for Non-SFI binary, but, in some
cases, there are small diffs which need to be guarded by #ifdef.
In "Non-SFI build binary" mode, __native_client_nonsfi__ is defined.
This CL introduce OS_NACL_SFI and OS_NACL_NONSFI which can be used among the
chrome code, to slightly simplify the #ifdef conditions.

This CL is the follow up of crrev.com/659243002

BUG=358465
TEST=Ran trybot.

Review URL: https://codereview.chromium.org/664373003

Cr-Commit-Position: refs/heads/master@{#301603}
diff --git a/PRESUBMIT.py b/PRESUBMIT.py
index 8ec7f532..2c405655 100644
--- a/PRESUBMIT.py
+++ b/PRESUBMIT.py
@@ -265,6 +265,8 @@
     'OS_LINUX',
     'OS_MACOSX',
     'OS_NACL',
+    'OS_NACL_NONSFI',
+    'OS_NACL_SFI',
     'OS_OPENBSD',
     'OS_POSIX',
     'OS_QNX',