[Media] Fix video encoder crash

This patch fixes a serious bug where an access error occurs due to
attempting to dereference an invalid VpxVideoEncoder instance in
the MediaVideoEncoderWrapper class.

The error occurred because, when swapping out encoders due to a size
change in `MediaVideoEncoderWrapper::ConstructEncoder`, the old encoder
instance is inadvertently deleted on the MAIN thread instead of being
posted to the VIDEO thread, so any decode tasks that refer to it
become invalid.

The solution is to stop using a DeleteSoon() and instead use the
base::OnTaskRunnerDeleter deleter with the VIDEO cast environment
task runner.

Bug: 427251884
Change-Id: I98723cfb764c40b35a3157de7c6d694686631dcd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6681751
Reviewed-by: Eugene Zemtsov <eugene@chromium.org>
Commit-Queue: Jordan Bayles <jophba@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1479609}