[extensions] Fix dcheck failures in getV8Statistics

HeapObjectIterator creates a SafepointScope which requires the heap to
allow garbage collection. This collides with the outer
DisallowGarbageCollection scope. HeapObjectIterator already ensures
there is no allocation during its lifetime, so there is no need to
create an outer DisallowGarbageCollection scope.

Code::source_position_table requires their kind not equals to
CodeKind::BASELINE.

This also exposes the statistics extension through flag
--expose-statistics.

Bug: v8:12657
Change-Id: I1bf11cf499285a742dd99ec8c228ebc36152b597
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3496552


Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Chengzhong Wu <legendecas@gmail.com>
Cr-Commit-Position: refs/heads/main@{#79425}