This is an unfortunate restriction imposed by Worklist::kMaxNumTasks
for now.

This CL unbreaks tests for developers. The CQ didn't catch this breakage
because bots have 8 cores and concurrent marking uses num_cores-1.

R=hpayer@chromium.org

TEST=All tests passed on dev machine (was super broken without this change)
NOTRY=True (to unbreak devs)

Bug: v8:7477,chromium:812178
Change-Id: I644613857c74d1ae00965f3e6d1d7692a4303062
Reviewed-on: https://chromium-review.googlesource.com/931461
Commit-Queue: Gabriel Charette <gab@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51473}