This CL introduces a ScopedPtrList that's a view over an underlying ZonePtrList
buffer. Whenever a ScopedPtrList is the top-of-stack list, you can add values
through it, which will add them to the end of the buffer. Once the list is
done, you can copy out the values to a real ZonePtrList. That way you do not
need to guess what the required size of the list is, and you get better cache
locality.

Change-Id: I2d229d73bb25bbb450ae5b6767ab100abad2b3a3
Reviewed-on: https://chromium-review.googlesource.com/c/1296458
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56939}