Reland "Implement WebContents discard by clearing the primary frame document"

This is a reland of commit ce4ea2692091b299b26cc7967f9e1b980c583e09

See diff against PS1.

Addressed timeout issue in DiscardClearsDedicatedWorkers test.
Destruction of blink::DedicatedWorker post Discard() relies on
async GC sweep in V8, which can result in timeouts on bots.

Updated extension tab loading status to report unloaded when the
tab is discarded. The tab will retain its render process when
discarded with the new mechanism (consistent with previous behavior).

Original change's description:
> Implement WebContents discard by clearing the primary frame document
>
> See go/discard-frame-tree for additional details and to contribute
> to discussion.
>
> This carries on from fdoray's prototype (crrev.com/c/4060064) and
> erik's follow up (crrev.com/c/5454457).
>
> This CL introduces a new discard mechanism that avoids needing
> to replace the tab's WebContents.
>
> Discard works by clearing the primary frame tree, done via replacing
> Blink's primary document. This frees resources associated with the
> primary frame and clears away any child frames attached to the
> frame tree.
>
> Discard does not result in creation of any session entries and does
> not emit navigation-related events
>
> Key changes:
>  - Update TabLifecycleUnit::FinishDiscard to no longer create
>    a empty WebContents for the discard operation. Instead discard
>    is implemented via replacing the primary blink document
>
>  - The tab thumbnail implementation has been updated to handle the
>    new discard mechanism
>
>  - OneTimePermissionsTrackerHelper was updated to leverage the
>    existing WebContents observation instead of relying on
>    TabLifecycleObserver
>
>  - Multiple test updates / fixes to reflect discard API changes
>    and address incorrect assumptions (active tabs cannot be
>    discarded)
>
> Notifications for discard events with both the old and new
> WebContents from the original API remain. These can be removed or
> refactored in the follow up CLs.
>
> PERFETTO_TESTS=`autoninja -C out/Default perfetto_diff_tests &&
> out/Default/bin/run_perfetto_diff_tests`
>
> Bug: 347770670
> Change-Id: I32e87b2b992914a7ae0e0bbdddf78f1b27c5773b
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5637909
> Commit-Queue: Thomas Lukaszewicz <tluk@chromium.org>
> Reviewed-by: Nicolás Peña <npm@chromium.org>
> Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
> Reviewed-by: Victor Vianna <victorvianna@google.com>
> Reviewed-by: Florian Jacky <fjacky@chromium.org>
> Reviewed-by: David Pennington <dpenning@chromium.org>
> Reviewed-by: Rakina Zata Amni <rakina@chromium.org>
> Reviewed-by: Daniel Cheng <dcheng@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#1343920}

Bug: 347770670
Change-Id: Idba13fec3c375c9f88727826386729963bef9166
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5799663
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Reviewed-by: Victor Vianna <victorvianna@google.com>
Reviewed-by: Erik Chen <erikchen@chromium.org>
Commit-Queue: Thomas Lukaszewicz <tluk@chromium.org>
Reviewed-by: Francois Pierre Doray <fdoray@chromium.org>
Reviewed-by: Florian Jacky <fjacky@chromium.org>
Reviewed-by: Nicolás Peña <npm@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Rakina Zata Amni <rakina@chromium.org>
Reviewed-by: Emilia Paz <emiliapaz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1345153}