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This registers an IRT interface in Chromium instead of using the one provided
by NaCl. This reuses the ManifestServiceChannel used for providing
irt_open_resource() in non-SFI mode.

In this change, the Chromium implementation of NACL_IRT_RESOURCE_OPEN_v0_1
takes precedence over the one supplied by NaCl (which is SRPC-based).

The SRPC-based codepath in service_runtime.cc needs to be kept because the
PNaCl translator doesn't have the IRT available yet. I've added a check to
enforce that's the only user of that codepath.

BUG=394130
TEST=Manually tested with a file token that didn't resolve with a local patch
that forced GetFilePath to fail in nacl_process_host.cc and confirmed
that URLLoader* still passed.
CQ_EXTRA_TRYBOTS=tryserver.chromium.linux:linux_rel_precise32
R=dmichael@chromium.org, mseaborn@chromium.org, tsepez@chromium.org

Review URL: https://codereview.chromium.org/418423002

Cr-Commit-Position: refs/heads/master@{#294208}