To support compaction of backing stores in blink, we need to distinguish
custom spaces holding backing stores from other custom spaces.
Custom space compactablity is explicitly declared as an enum value and
propagated to BaseSpace as a bool flag.

Note that even if/when general compaction is implemented/enabled for
normal pages we will still need such a marking for supporting
non-compactable custom spaces.

Bug: v8:10990
Change-Id: I165a0268ded121e91399834a4091e88e57f2565c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2449973
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70345}