Read-only heap sharing clears heap_ in read-only memory chunks because
ReadOnlySpace is shared between multiple isolates.

Bug: v8:7464
Change-Id: I821c94303ab3710c279e6c11a8ca8537aac0d0af
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1642809
Commit-Queue: Maciej Goszczycki <goszczycki@google.com>
Reviewed-by: Dan Elphick <delphick@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61987}