This changes DebugObjectCache to be a vector of Handles rather than
tagged pointers, meaning it's not GC-safe.

This will allow PrintStack to allocate memory if required (if for
instance source positions must be regenerated).

Bug: v8:8834, v8:8510
Change-Id: Ieec9a827af9abbcb9b5b237d79984eedf0cdcc57
Reviewed-on: https://chromium-review.googlesource.com/c/1494755Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59952}