Move ownership of PrefetchResponseReader to PrefetchContainer

This CL makes `PrefetchResponseReader` owned by
`PrefetchContainer::streaming_loaders_` instead of
`PrefetchStreamingURLLoader`, and thus decouples the
ownership/lifetime of `PrefetchResponseReader` from
`PrefetchStreamingURLLoader`.

After start serving, `PrefetchStreamingURLLoader` and
`PrefetchResponseReader` are self owned and kept alive by
prefetching-side and serving-side mojo channels, respectively.
`PrefetchResponseReader` doesn't keep alive its `PrefetchStreamingURLLoader`, because after the prefetching
URLLoader is finished all the data needed for serving is
stored in `PrefetchResponseReader`.

Design doc:
https://docs.google.com/document/d/133Xwdh0sEnODHHOdt5QMdFMdfanTJ01UecCupjOjdZo/edit#heading=h.6cyajgj47dx
(Implementation Design (Phase 1) section)

Bug: 1449360
Change-Id: I2d8063208bca092011b03ab0c9c46e6c418a50ea
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4611970
Commit-Queue: Hiroshige Hayashizaki <hiroshige@chromium.org>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Reviewed-by: Max Curran <curranmax@chromium.org>
Reviewed-by: Ryan Sturm <ryansturm@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1160453}