This makes sense since the buffered data is only used by
the echo subtraction method. Furthermore, it simplifies the
upcoming modifications to the echo subtraction method since
the way the buffering is done can then be specific for the
echo subtraction implementation used.
The change is bitexact and this was verified using a fairly
extensive bitexactness suite.
BUG=
Review URL: https://codereview.webrtc.org/1639773002
Cr-Commit-Position: refs/heads/master@{#11547}
1147b75b52