For the sake of incremental development it makes sense to still allow
serialization to happen even when the broker is in mode 'kSerialized'.
To locally obtain the stricter behavior, I'm adding the temporary flag
--strict-heap-broker.

Bug: v8:7790
Change-Id: Ie92f4f13d76bac9ce1f98bdd245829984d436d77
Reviewed-on: https://chromium-review.googlesource.com/1156603
Commit-Queue: Georg Neis <neis@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54824}