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Add a copy of the existing SRPC-based implementation --
native_client/src/untrusted/irt/irt_pnacl_translator_{compile,link}.c
-- to the Chromium side.  This will allow the two implementations to
be independently changed to remove use of SRPC.

In copying this code, I have made some changes to follow the Chromium
style:

 * Switching to C++ and using C++ comments
 * Using anon namespaces instead of "static"
 * "*" spacing (using clang-format)
 * "FooBar" function naming instead of "foo_bar"
 * Using the Chromium copyright notice

BUG=302078
TEST=e.g. NaClBrowserTestPnacl.PPAPICore in browser_tests
  (also manually tested with NaCl-side interface disabled, to ensure the
  Chromium-side one is really being used)

Review URL: https://codereview.chromium.org/1240343004

Cr-Commit-Position: refs/heads/master@{#339825}