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This is part of the recording service work needed for capture mode.
(See go/simple-recording-service).

This CL defines the base APIs for audio encoders, and ports the PCM
and Opus implementations from blink to //media. A follow-up CL should
be to migrate blink's media recorder to use these new encoders.

BUG=1131275, 1126586
TEST=Added tests at media_unittests --gtest_filter=*AudioEncodersTest*
     Also, Manually tested both encoders in my WIP recording service
     CL, and the output captured webm file contains recorded audio
     that plays back successfully.

Change-Id: Iea132162d319a5b580e6b447e3d5658fbf731de6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2426146
Commit-Queue: Ahmed Fakhry <afakhry@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#811552}