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Adds some mojo interfaces for configuring audio processing, intended for
both WebRTC and system-level echo cancellation, though the latter will
be less configurable.

There are two parts to this change:
1. AudioProcessingSettings in media/audio, used to configure audio
   processing when done in the audio service. At one point, the similar
   AudioProcessingParameters in content will be replaced by this one.
2. AudioProcessorControls in media/webrtc; an interface used to control
   auxiliary audio processor functionality, specifically getting stats
   and starting/stopping AEC dumps.

For an outline of the project this CL is part of, see: https://docs.google.com/document/d/1u4POff_ts_1LE3WDLA_wDDFnUswdlsuHL5DsiTE0a3U/edit?usp=sharing
It's accessible to everyone @chromium.org.

Bug: 851959
Tbr: tommi@webrtc.org
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;luci.chromium.try:win_optional_gpu_tests_rel
Change-Id: I0c8c934b35b93aa728a0d1b60ae76063e925ee6c
Reviewed-on: https://chromium-review.googlesource.com/1167848
Commit-Queue: Tommi <tommi@chromium.org>
Reviewed-by: Tommi <tommi@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583239}