AudioConverter: Express delay in frames rather than msec.

This is a todo from https://codereview.chromium.org/1687213002/

We already use frames on input and output sides of AudioConverter,
so this unifies things and deletes some conversions.

It also paves the way for another cleanup CL:
https://codereview.chromium.org/2004963002/

BUG=587522

Review-Url: https://codereview.chromium.org/2004283002
Cr-Commit-Position: refs/heads/master@{#396256}
34 files changed