Revert "[PA] Replace largest_active_bucket_index_ with active_bucket_count_"

This reverts commit 75f790d6fca4e88282c5d68c8f0faa40c45022c7.

Reason for revert: Parent CL is suspected cause of memory regression

Failure Link: https://pinpoint-dot-chromeperf.appspot.com/job/157bfe49690000

Original change's description:
> [PA] Replace largest_active_bucket_index_ with active_bucket_count_
>
> This CL renames `largest_active_bucket_index_` to `active_bucket_count_`
> in `ThreadCache` and `ThreadCacheRegistry`, and optimizes hot-path
> validity and bucket bounds checks in PartitionAlloc (`bucket_index <
> tcache->active_bucket_count()`).
>
> This makes use of zero-initialized `ThreadCache` safer - it simply
> denotes there is no active bucket and routes every allocation to the
> global root instead.
>
> TAG=agy
> CONV=80980e77-c356-4c8b-a00b-3a2c47533fbb
>
> Change-Id: Ie599f94695b8f5d1662da26f5efd2ad9fa6b021a
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/8254723
> Reviewed-by: Takashi Sakamoto <tasak@google.com>
> Commit-Queue: Mikihito Matsuura <mikt@google.com>
> Cr-Commit-Position: refs/heads/main@{#1690469}

Change-Id: I399006f53b684b1575abf5ffe32eae791e4fa03f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/8408627
Owners-Override: Anna Tsvirchkova <atsvirchkova@google.com>
Bot-Commit: rubber-stamper@appspot.gserviceaccount.com <rubber-stamper@appspot.gserviceaccount.com>
Commit-Queue: Alex Mitra <alexmitra@chromium.org>
Reviewed-by: Mateusz Zbikowski <zbikowski@google.com>
Cr-Commit-Position: refs/heads/main@{#1698072}
NOKEYCHECK=True
GitOrigin-RevId: 7722d5682e81c10e435d72e0b4a6ed1fc67853a9