This CL simplifies and extends the implementation of Managed<T>
and now uses a std::shared_ptr<T> underneath in order to offer
cross-isolate management of C++ allocated memory.

R=mstarzinger@chromium.org
CC=ulan@chromium.org

Bug: v8:7424
Cq-Include-Trybots: luci.v8.try:v8_linux_noi18n_rel_ng
Change-Id: Id43a26f565677e8c9cdfd73810568d4f2b1871fe
Reviewed-on: https://chromium-review.googlesource.com/1028190Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52820}