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- Add mix & linearize AudioPostProcessor hooks.
- Use delay from AudioPostProcessor::ProcessFrames to update timing estimates.
- Allow configuring mixing multiple device_id's in a single AudioPostProcessor.
- Pass Cast Volume to PostProcessors (rather than raw multiplier).
- Add unittest for assignment of PostProcessors + delay accounting.

BUG=internal/36299959
TEST=cast_alsa_cma_backend_unittests

Change-Id: I5503f7de39d0ac502b8e861322162fee9aade8dd
Review-Url: https://codereview.chromium.org/2847673002
Cr-Commit-Position: refs/heads/master@{#468399}