Pepper: Add a Pepper+NaCl+Mojo test.

Pepper: Add a Pepper+NaCl+Mojo test.

This change injects a NaClDesc into an FD available to the untrusted
plugin for making Mojo calls. This is the foundation for adding support
for Mojo to Pepper plugins.

It's important to note, from a security perspective, that this exposes
implementations of the functions in mojo/public/c/system that run
outside the NaCl sandbox to plugins.

I have a working prototype change where a Pepper plugin makes calls to a
Mojo service implementated in the browser which uses the logic here.
However, this seems like the best place to get started checking things
in.

This interface doesn't go through the IRT for performing Mojo calls;
that will be added in a future change.

This test works on Linux, Mac, and x86-32 Windows. x86-64 Windows
support will be added when http://crbug.com/440012 is resolved.

BUG=414804

Review URL: https://codereview.chromium.org/701273002

Cr-Commit-Position: refs/heads/master@{#309722}