[media] Support parent thread decode in VdaVideoDecoder.

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: I48b205e7b20cf512e953d5992be6ed507add688b
Reviewed-on: https://chromium-review.googlesource.com/1217716
Reviewed-by: Frank Liberato <liberato@chromium.org>
Commit-Queue: Dan Sanders <sandersd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590479}
3 files changed