After closing a handle scope, and when allocating a new segment in a
zone, treat that memory as uninitialized in MSan. This will hopefully
catch more errors than handle zapping, which needs to be enabled
explicitly.

R=ahaas@chromium.org

Bug: v8:7570
Change-Id: Ie3be07434bed878fb607a522787514421f397197
Reviewed-on: https://chromium-review.googlesource.com/1046657
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53089}