HttpStreamPool::AttemptManager: Rework lifetimes.

Previously, the AttemptManager was bound to all HttpStreamPool::Jobs
that used it for the lifetime of those Jobs. Each AttemptManager kept
a set of Jobs that had been notified of completion, just for this
purpose. Also as a result, any consumer that didn't promptly clean up
completed Jobs (like HttpStreamPoolJob::Controller, on failure, when
there were two jobs) could impact lifetime of the AttemptManager and
the owning Group.

The CL modifies the relationship so that Jobs are disassocited with
the AttemptManager on completion (or cancellation), removing the
extra job tracking logic. Since AttemptManager always post takes to
tell their Group to delete them asynchronously, anyways, this is a
safe change to make.

The one issue that cropped up is that a lot of tests poke at
AttemptManager in the previously caller-controllable window
between Job completion and AttemptManager destructruction. That
window still exists, but it's now more difficult to write tests
around it. This required pretty extensive test refactors.

I'd like to have AttemptManagers be deleted synchronously on
completion down the line, but that's an issue for another day.

Bug: 346835898
Change-Id: I33d3ba4389f32fe35cff131e5799de32e6a2e60e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7030587
Reviewed-by: Kenichi Ishibashi <bashi@chromium.org>
Commit-Queue: mmenke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1530418}