[Extensions] Remove always true assumption when unregistering a worker.

Before this change it seemed that in
ServiceWorkerTaskQueue::DidUnregisterServiceWorker() an extension was
not always deactivated when the unregistration completes, but this is
always done in ServiceWorkerTaskQueue::DeactivateExtension().
DeactivateExtension() is the only caller of
DidUnregisterServiceWorker().

As part of this we needed to update the
WorkerUnregistrationFailureStatus metrics since they now emit at
different times.

OBSOLETE_HISTOGRAM[Extensions.ServiceWorkerBackground.WorkerUnregistrationFailureStatus_DeactivateExtension_NotCurrentActivation]=Deleted since it's not longer needed.
OBSOLETE_HISTOGRAMS=WorkerUnregistrationFailureStatus is replaced by WorkerUnregistrationFailureStatus2 since they now emit at different times.

Bug: 346732739
Change-Id: Iaa255e2e413e57bb3e89c9237cbab6f52bdc85b9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5762366
Reviewed-by: Emilia Paz <emiliapaz@chromium.org>
Commit-Queue: Justin Lulejian <jlulejian@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1337579}