Use the slimmer base::LeakyObject instead of base::LazyInstance.

R=tebbi@chromium.org

Bug: v8:8600
Change-Id: I71755db9fe3ea9c61be2cdf009a006947ef5560a
Reviewed-on: https://chromium-review.googlesource.com/c/1392203Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58544}