[media] Make MojoMediaLog implementation thread-safe.

Also plumbs the media log to VdaVideoDecoder, in preparation for
plumbing all the way to VDAs.

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: Id19d3edf130ac6b8aae8a86b9b7afbf5e464eafb
Reviewed-on: https://chromium-review.googlesource.com/1036590
Commit-Queue: Dan Sanders <sandersd@chromium.org>
Reviewed-by: Xiaohan Wang <xhwang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#555181}
8 files changed