The check is a simple shortcut, but this is not safe in multithreading.

In a multi-threaded situation, if a CodePageCol**Scope is open while
a CodeSpaceMem**Scope is already opened, the result is a noop.
If the latter finishes first, then we would decrement a wrong
depth in ~CodePageCollectionMemoryModificationScope.

Bug: v8:12054
Change-Id: I7e1016628ffbd37b343ea130eb8d7d8e60abec98
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3275562Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Commit-Queue: Victor Gomes <victorgomes@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77849}