Make PrefetchService receive PrefetchRequest, not PrefetchContainer

Before this CL:

1. A `PrefetchContainer` is created by the users of `PrefetchService`
   and then passed to `PrefetchService`.
2. The `PrefetchContainer` is either added to
   `PrefetchService::owned_prefetches_`, or destroyed immediately when
   it is merged into an existing `PrefetchContainer`.

After this CL:

1. A `PrefetchRequest` is created by the users of `PrefetchService`
   and then passed to `PrefetchService`.
2. When it is not merged into an existing `PrefetchContainer`, a new
   `PrefetchContainer` is created and added to
   `PrefetchService::owned_prefetches_`.

In order to:

- Avoid transient `PrefetchContainer` construction followed by
  its immediate destruction.
- Make `PrefetchContainer` always owned by
  `PrefetchService::owned_prefetches_` in non-test code.
- Create `PrefetchContainer`s only from `PrefetchService` in non-test
  code.
- Clarify that `PrefetchRequest`s are merged into an
  `PrefetchContainer`.

Bug: 437631382
Change-Id: I0c29d7f99eff21a090bd636eaf805392ddafb592
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6897472
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Commit-Queue: Hiroshige Hayashizaki <hiroshige@chromium.org>
Reviewed-by: Taiyo Mizuhashi <taiyo@chromium.org>
Reviewed-by: Ken Okada <kenoss@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1512056}