Includes unit tests for the post-processing step
flatten_regions_to_parts().

Bug: v8:5244
TBR: bmeurer@chromium.org, rossberg@chromium.org
Cq-Include-Trybots: master.tryserver.v8:v8_linux_noi18n_rel_ng
Change-Id: I306dd1721cc00c5820b061f14c4b6866f8d938f6
Reviewed-on: https://chromium-review.googlesource.com/529973
Commit-Queue: Josh Wolfe <jwolfe@igalia.com>
Reviewed-by: Daniel Ehrenberg <littledan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46369}