[media] Retain VideoToolbox session during seeks

VideoToolboxVideoDecoder was destroying the decompression session on
Reset(), which avoided having an active session while waiting for
the network, and ensured that the specific pattern of seeks could not
affect decoding behavior.

Unfortunately recreating the decompression session is slow (~30ms),
resulting in choppy behavior when dragging the seek slider continuously.

After this change, VideoToolboxVideoDecoder will always retain the
current decompression session after a Reset().

Bug: 325814596
Change-Id: Iea0b969fa936488c54a22dc5059d7323e7893b23
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5328872
Reviewed-by: Thomas Guilbert <tguilbert@chromium.org>
Commit-Queue: Dan Sanders <sandersd@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1266024}