1) Pass {std::shared_ptr} by reference if a copy might not be needed.
   This applies both to accessors as well as constructing methods.
   This change often saves one atomic increment and decrement of the
   internal reference counter.
2) Use {std::make_shared} directly to improve memory management of the
   {std::shared_ptr}. This saves one dynamic memory allocation on all
   known implementations.

R=titzer@chromium.org

Bug: v8:8834
Change-Id: I1a951deb135082745885bc981662a8d6d6fb1a71
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1532333Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60425}