This removes the static ReadOnlyHeap::Instance method replacing it with
PopulateReadOnlySpaceStatistics on the way to removing the global
ReadOnlyHeap object.

Bug: v8:10454
Change-Id: Ic78c898ff99c6a7dac023d2b5230fbbbf6f36f46
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2179805
Commit-Queue: Dan Elphick <delphick@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Auto-Submit: Dan Elphick <delphick@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67545}