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}