C++ readability review for ajm.

As part of the review, refactored AudioConverter into internal derived
classes, each focused on one type of conversion. A factory method
returns the correct converter (or chain of converters, via
CompositionConverter).

BUG=b/18938079
R=rojer@google.com

Review URL: https://webrtc-codereview.appspot.com/35699004

Cr-Commit-Position: refs/heads/master@{#8322}
git-svn-id: http://webrtc.googlecode.com/svn/trunk@8322 4adac7df-926f-26a2-2b94-8c16560cd09d
6 files changed