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
Review URL: https://codereview.chromium.org/
288773002
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@270887 0039d316-1c4b-4281-b951-d872f2087c98