[media] Log to MediaLog from VTVDA.

This moves logging of bitstream errors that were previously in the GPU
log to the media log.

To support async destruction correctly, VdaVideoDecoder now implements
the MediaLog interface and handles thread hopping.

Also includes some additional cleanups in VTVDA: improved log messages,
increase the number of requested picture buffers to match recent changes
in DXVAVD, and returns no supported profiles if VideoToolbox fails to
initialize.

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: I7f13deb9e92fdeeeac60479e17081e72c5f22066
Reviewed-on: https://chromium-review.googlesource.com/1038609
Commit-Queue: Dan Sanders <sandersd@chromium.org>
Reviewed-by: Xiaohan Wang <xhwang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#556164}
7 files changed