This was disabled in 2014 (https://crrev.com/267383002), together with
a comment about a broken serializer. The conditional v8 initialization
was since then moved back to be unconditional, but the TearDown was
never restored.
Now we need it for wasm, since during tear down the wasm engine
synchronizes on all background compile jobs. Omitting this leads to
uses of the disposed platform (see https://crrev.com/c/1429861).

R=mstarzinger@chromium.org

Bug: v8:8689, v8:7921, v8:8725
Change-Id: Ia24f746094f38fc6ce349532587b622384379125
Reviewed-on: https://chromium-review.googlesource.com/c/1430059
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59032}