Followup after https://chromium-review.googlesource.com/c/v8/v8/+/652427
which triggered FORWARD_DECLARE macro redefinition warnings/errors.  Let's
make sure to undef this macro at the end of source files that create it.

Bug: chromium:746958
Change-Id: Ie70743c836db45eb51f412a0d6359f0f667319d3
Reviewed-on: https://chromium-review.googlesource.com/654657
Commit-Queue: Mostyn Bramley-Moore <mostynb@opera.com>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47866}