[tasks] Make idle request timeout cancelable

Using a timer makes tasks cancelable, which should reduce
OOM related to delayed tasks queue.

Change-Id: I6ba4b7101f2ff694303a366d7f1d4b882a668da9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5449850
Reviewed-by: Scott Haseley <shaseley@chromium.org>
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Reviewed-by: Francois Pierre Doray <fdoray@chromium.org>
Reviewed-by: Mason Freed <masonf@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1287514}