Since we're not winning anything by changing the result between
processors on the main process, reduce is noop there and result is
immutable.

Bug: v8:6917
Change-Id: Ieb282e7abd4ab31162aee6b52493a6e1b6a25109
Cq-Include-Trybots: luci.v8.try:v8_linux64_fyi_rel_ng
Reviewed-on: https://chromium-review.googlesource.com/878239
Commit-Queue: Michał Majewski <majeski@google.com>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50784}