Each instance of {DeferredHandles} only stores a single handle anyway,
so turning them into global handles saves memory.

R=ahaas@chromium.org

Bug: v8:7921, v8:8423
Change-Id: I75e0cc6ecb6d0337f1695e6bfd4b5fdcb8a2039c
Reviewed-on: https://chromium-review.googlesource.com/c/1402715
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59331}