Rather than using an ad-hock ownership model for ast_value_factory, use a
shared_ptr.

BUG=v8:5203

Change-Id: I5f2a573c8b175a3138ad8b01aa78bddadd16e6d3
Reviewed-on: https://chromium-review.googlesource.com/582628
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46874}