D8 workers were leaking all messages due to a missing HandleScope
within the message loop. Add one to fix leaks.

R=verwaest@chromium.org
BUG=v8:9524

Change-Id: I2f2f6ba0b8c58bf38bf68b7464dffdf1d826a29d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1724212Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62992}