[media] Support DecoderBuffers in CrOS VDAs.

DecoderBuffers abstract over SHM and heap allocations. This is necessary
to efficiently use these VDAs via MojoVideoDecoder.

Bug: 522298
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: I1d27410a8fcc4da2786696fba802cf956893f91d
Reviewed-on: https://chromium-review.googlesource.com/1175097
Commit-Queue: Dan Sanders <sandersd@chromium.org>
Reviewed-by: Hirokazu Honda <hiroh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590842}
8 files changed