If the main thread (or multiple main threads, or different isolates)
executes compilation units, it uses task id 0. This id will also be used
by the first worker thread. Avoid this by shifting the ids of worker
threads by one.

R=thibaudm@chromium.org
CC=etiennep@chromium.org

Bug: v8:11005
Change-Id: I3beb8a5716112d9466c5b0296ab4ed1f2cf20519
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2471378Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70613}