[optimization guide] Ensure model file is always deleted on correct sequence.

TFLiteModelExecutor attempts to delete its MemoryMappedFile on a
sequence that allows blocking. However, in the case where the reply
to the "PostTaskAndReplyWithResult" in "LoadModelFile" cannot run
(e.g. during shutdown), the MemoryMappedFile may be deleted on the
main thread, which doesn't allow blocking. This CL uses
base::OnTaskRunnerDeleter to ensure that deletion always happen
on the correct sequence (even when deletion is not explicit).

Bug: 368093394
Change-Id: I9d41830a7ebcc9749f1252a7f779c5b87a399468
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5881895
Auto-Submit: Francois Pierre Doray <fdoray@chromium.org>
Commit-Queue: Sophie Chang <sophiechang@chromium.org>
Reviewed-by: Sophie Chang <sophiechang@chromium.org>
Commit-Queue: Francois Pierre Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1358862}