Extends the functionality of Managed<T> to track an estimated size
for the external memory associated with an instance of Managed<T>
in order to allow for proper accounting in the garbage collector.

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

Cq-Include-Trybots: luci.v8.try:v8_linux_noi18n_rel_ng
Change-Id: I8c49c6245eaf267c9264ebb93b43d5dfbf4671fd
Reviewed-on: https://chromium-review.googlesource.com/1076332
Commit-Queue: Ben Titzer <titzer@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53433}