Set number of decode threads equal to number of available cores and
limit the maximum value to the maximum number of tiles possible for
HD resolution.
Bug: webrtc:9829, b/117291409
Change-Id: Ib5ccd5cc412011d4438258491efc060cdd050fc7
Reviewed-on: https://webrtc-review.googlesource.com/c/104064
Reviewed-by: Erik Språng <sprang@webrtc.org>
Commit-Queue: Sergey Silkin <ssilkin@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25059}