This wraps CagedHeap in #if defined(CPPGC_CAGED_HEAP)

Bug: v8:12231
Change-Id: I8407abdd88c6a96b6c79c08e7039b5eb6f8175f9
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3695384Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Auto-Submit: Anton Bikineev <bikineev@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81010}