Non-SFI Mode: Build ipc/ library by PNaCl toolchain for nacl_helper_nonsfi.

This CL is to build ipc/ for nacl_helper_nonsfi.
The library is similar to ipc_nacl, but slightly different:
- The IPC::Channel should use ChannelPosix rather than ChannelNaCl,
  as it runs under linux directly.
- Some features of ChannelPosix cannot be compiled by PNaCl toolchain for
  Non-SFI build, but these are not necessary for nacl_helper_nonsfi.
  These are dropped by "ifdef" guard.
Note that this library is not used yet, but should be built successfully.

BUG=358465
TEST=Ran trybot. Implement nacl_helper_nonsfi on top of this CL, and made sure it works.

Review URL: https://codereview.chromium.org/659243002

Cr-Commit-Position: refs/heads/master@{#301037}

• components/nacl_nonsfi.gyp[diff]
• ipc/ipc_channel.cc[diff]
• ipc/ipc_channel.h[diff]
• ipc/ipc_channel_posix.cc[diff]
• ipc/ipc_channel_proxy.cc[diff]
• ipc/ipc_channel_proxy.h[diff]
• ipc/ipc_nacl.gyp[diff]