[Gardener] Revert "[Extensions] Restructure extensions::ProcessMap"

This reverts commit ff64716bd592665848ed4277d33e57c2ccabe04b.

Reason for revert: Looks like broke tree:
https://ci.chromium.org/ui/p/chromium/builders/ci/linux-cfm-rel/56731/overview

Original change's description:
> [Extensions] Restructure extensions::ProcessMap
>
> ProcessMap was originally implemented to support a many-to-many
> relationship: an extension could potentially occupy multiple processes,
> and vice versa -- multiple extensions could run in a single process.
>
> Nowadays, with all the perks we get from site isolation, it no longer
> makes sense to support the many-to-many relationship between extensions
> and processes. However, it's worth noting that we're unable to model
> this as a one-to-one relationship for a number of reasons (such as
> supporting hosted apps that occupy multiple processes at once or
> supporting extensions that have both COI and non-COI frames), so the
> suggested model is one-to-many: an extension can run in multiple
> processes in parallel, but there cannot be more than one extension in a
> single process.
>
> The last known process sharing case for a single extension were hosted
> apps that covered different paths of the same site (when over the
> process limit); this case went away in crrev.com/c/5187071.
>
> Bug: 329445684, 338476400
> Change-Id: I26af57aba2f04d8b2a9e541958aa140c2d2f1b44
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5370192
> Reviewed-by: Devlin Cronin <rdevlin.cronin@chromium.org>
> Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
> Reviewed-by: David Trainor <dtrainor@chromium.org>
> Commit-Queue: Andrew Rayskiy <greengrape@google.com>
> Cr-Commit-Position: refs/heads/main@{#1295889}

Bug: 329445684, 338476400
Change-Id: I40091b3f73019d5b3c2ff7513192d9fb3901abd3
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5513963
Commit-Queue: Michelle Chen <michellegc@google.com>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Owners-Override: Michelle Chen <michellegc@google.com>
Cr-Commit-Position: refs/heads/main@{#1295896}