Forcing it seems unnecessary and hinders concurrent compilation.

Also turn a related condition into a CHECK.

Bug: v8:7790
Change-Id: Ie7d7e47e6bc2743d742cfe07e65436c1b8b994f3
Reviewed-on: https://chromium-review.googlesource.com/1177705Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55164}