v8_debug_helper attempts to flag known object pointers when it can
recognize them, even if the memory pointed to is not available in the
crash dump. In ptr-compr builds, the first pages of the map space,
read-only space, and old space are always at the same offsets within the
heap reservation region, so we can more easily detect known objects.

Bug: v8:9376
Change-Id: I04e0d2357143d753f575f556e94f8fd42ce9d811
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1783729
Commit-Queue: Seth Brenith <seth.brenith@microsoft.com>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63624}