NaCl: Expose NaClApp to the embedding layer.

NaCl: Expose NaClApp to the embedding layer.

This change exposes NaClApp and separate methods for loading and starting a
module to the embedding API.

It was initially proposed that a single method in the embedding API would take
care of loading and starting a module. I found this difficult to implement in
practice because the following 3 operations must occur in order:
  - Load NaCl module.
  - Initialize reverse service.
  - Start NaCl module.

After the reverse service is moved to Chromium IPC from SRPC, this may be
easier to manage, but for now, I'd like to have two separate methods for
loading and starting a NaCl module.

I've tested this change against the current HEAD of chrome, and with a
prototype chrome change that calls the two methods introduced in this API.

Chrome change: https://codereview.chromium.org/131413009/

BUG=http://crbug.com/333950

[teravest, 2014-02-06 16:33:38]

ping?

It looks like you're making some changes in a similar area, and I'd like to get
this embedding API nailed down. It looks like I'll have to move this to public/
anyway, but I really want to get your feedback on how NaClApp creation should
look, and how LoadModule()/StartModule() should be exposed.