Pepper: Simplify manifest download logic.

Pepper: Simplify manifest download logic.

This cleans up some of the trusted plugin logic for requesting a NaCl manifest
and removes some interfaces in PPB_NaCl_Private, providing better
encapsulation.

This code is easier to follow after this change, as well, since more of the
logic is unified in ppb_nacl_private_impl.cc. This is part of a larger effort
to remove the trusted plugin.

BUG=239656
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=270887

[dmichael (off chromium), 2014-05-15 18:01:21]

lgtm

[teravest, 2014-05-15 18:03:12]

The CQ bit was checked by teravest@chromium.org

[commit-bot: I haz the power, 2014-05-15 18:05:37]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/teravest@chromium.org/288773002/1

[commit-bot: I haz the power, 2014-05-16 01:22:23]

Change committed as 270887