Prevent d8 from exiting while wasm background compilation is still
going on. This prevents the need to use the testRunner to execute
webassembly tests in d8.

R=yangguo@chromium.org
CC=ahaas@chromium.org

Change-Id: I86fb7ce260fc56ee87040742f77b0ff86b8fbd53
Reviewed-on: https://chromium-review.googlesource.com/964221Reviewed-by: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51972}