Fix MSVC warnings about value truncations, webrtc/common_audio/ edition.

This changes some method signatures to better reflect how callers are actually
using them.  This also has the tendency to make signatures more consistent about
e.g. using int (instead of int16_t) for lengths of things like vectors, and
using int16_t (instead of int) for e.g. counts of bits in a value.

This also removes a couple of functions that were only called in unittests.

BUG=3353,chromium:81439
TEST=none
R=andrew@webrtc.org, bjornv@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/23389004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7060 4adac7df-926f-26a2-2b94-8c16560cd09d