Revert "[sandbox] Increase ExternalPointerTable maximum capacity to 512MB"

This reverts commit d7fcbba8.

Reason for revert: Speculatively reverting because of blocked roll

Original change's description:
> [sandbox] Increase ExternalPointerTable maximum capacity to 512MB
>
> Bug: v8:10391
> Change-Id: I383e11bdccf6fcaf13f29d25e1404545067d313e
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3891249
> Reviewed-by: Igor Sheludko <ishell@chromium.org>
> Commit-Queue: Samuel Groß <saelo@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#83203}

Bug: v8:10391
Change-Id: Id3a374a1f87a6ec3889d2e81d169ad004744ca99
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3904605
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/main@{#83305}

include/v8-internal.h

@@ -232,7 +232,7 @@ static_assert(kSandboxMinimumReservationSize > kPtrComprCageReservationSize,
 // size allows omitting bounds checks on table accesses if the indices are
 // guaranteed (e.g. through shifting) to be below the maximum index. This
 // value must be a power of two.
-static const size_t kExternalPointerTableReservationSize = 512 * MB;
+static const size_t kExternalPointerTableReservationSize = 128 * MB;
 
 // The maximum number of entries in an external pointer table.
 static const size_t kMaxExternalPointers =
@@ -241,7 +241,7 @@ static const size_t kMaxExternalPointers =
 // The external pointer table indices stored in HeapObjects as external
 // pointers are shifted to the left by this amount to guarantee that they are
 // smaller than the maximum table size.
-static const uint32_t kExternalPointerIndexShift = 6;
+static const uint32_t kExternalPointerIndexShift = 8;
 static_assert((1 << (32 - kExternalPointerIndexShift)) == kMaxExternalPointers,
               "kExternalPointerTableReservationSize and "
               "kExternalPointerIndexShift don't match");