Revert part of r7561, "Refactor audio conversion functions."

Specifically, revert this part:

  "Remove hacks in AudioBuffer intended to maintain bit-exactness with
   the float path. The conversions etc. are now all natural, and
   instead we enforce close but not bit-exact output between the two
   paths."

But keep the conversion function rename, since that doesn't seem to be
causing problems.

R=tina.legrand@webrtc.org, bjornv@webrtc.org
TBR=andrew@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/24999004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7569 4adac7df-926f-26a2-2b94-8c16560cd09d