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}

• PRESUBMIT.py[diff]
• build/build_config.h[diff]
• ipc/ipc_channel.h[diff]
• ipc/ipc_channel_proxy.cc[diff]
• ipc/ipc_channel_proxy.h[diff]