[media] Add media::*Decoder::IsPlatformDecoder().

Add IsPlatformDecoder() API to media::VideoDecoder, and also to
media::AudioDecoder for symmetry. This value will be used by
media::DecoderStream to decide when to fall back to software decode for
low-resolution video.

Bug: 684792
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: I778ebdcaa84a9cabfc74be9729969db53b2aad7a
Reviewed-on: https://chromium-review.googlesource.com/1139097
Commit-Queue: Dan Sanders <sandersd@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575749}
12 files changed