Add a "disabled" mojo interface to libmojo.

Mojo support in Pepper plugins will be protected behind a command-line
switch to get some experience with integrating Mojo and Pepper before
exposing it to users.

This requires that we introduce a "disabled" version of the libmojo
interface, which won't successfully make any Mojo system API calls from
inside the NaCl sandbox.

Additionally, this adjusts the file descriptor number used for the
trampoline that exposes Mojo system API calls; the previous number
conflicted with existing uses in Chromium.

BUG=https://code.google.com/p/chromium/issues/detail?id=414804
R=yzshen@chromium.org

Review URL: https://codereview.chromium.org/755283003