GitLab

BUG=internal 29459582
BUG=internal 31162371
TEST=cast_alsa_cma_backend_unittests --gtest_filter=StreamMixerAlsaTest.*
     exercise stream volume API; listen for smooth volume transitions

Review-Url: https://codereview.chromium.org/2341783004
Cr-Commit-Position: refs/heads/master@{#419540}