Rename Managed::New to Managed::From (since it takes ownership of an
existing object), and re-introduce Managed::Allocate, which allocates
a new object and stores it in a Managed.

R=titzer@chromium.org

Change-Id: I20b0750697fbe7d56d3816b19919c31e389278b3
Reviewed-on: https://chromium-review.googlesource.com/645806Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47794}