Pepper: Use base::Callback in RequestNexeFd.
GetNexeFd() in ppb_nacl_private_impl will change soon to be an internal method,
not one exposed through the PPB_NaCl_Private interface. From inside Chromium,
base::Callback is much easier to use than PP_CompletionCallback or
TrackedCallback.
This is one of a series of changes to refactor the pexe downloading logic in
the trusted plugin.
BUG=370556
Review URL: https://codereview.chromium.org/
380773002
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@282112 0039d316-1c4b-4281-b951-d872f2087c98