[media] Add more documentation to VideoDecoderConfig members.

Change-Id: I3704ec5c831498aa876ae3cea6ca458d77c0d275
Reviewed-on: https://chromium-review.googlesource.com/c/1401608
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Reviewed-by: Hirokazu Honda <hiroh@chromium.org>
Commit-Queue: Dan Sanders <sandersd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#621311}
1 file changed