GetMaxBackgroundTasks should return 0 in predictable mode, since
compilation is done in the foreground.

R=clemensh@chromium.org

Change-Id: I4a617cadb53ca91ee21e40c46a93d54e2a1ceb8b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1789301
Commit-Queue: Thibaud Michaud <thibaudm@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63600}