Encoders may use "phase inversion" to improve stereo compression. But
this creates artifacts if the decoded stereo is later down-mixed to
mono. This CL sets a flag on the decoder to disable the phase inversion
decode step whenever the HW is detected as mono.

Bug:806219
TBR:dalecurtis@chromium.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: Icff0387c7fc0fc0e0e31657176721cee5abf6fd9
Reviewed-on: https://chromium-review.googlesource.com/1038617Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Reviewed-by: Chrome Cunningham <chcunningham@chromium.org>
Commit-Queue: Chrome Cunningham <chcunningham@chromium.org>
Cr-Commit-Position: refs/heads/master@{#555257}