ReduceCompilationPriority takes a lock now.

Original message:
This Cl changes the priority of baseline compilation from kUserVisible
to kUserBlocking. Once baseline compilation finishes, the priority is
reduced to kUserVisible. The reason for using kUserBlocking is that
thereby TurboFan compilation cannot block Liftoff compilation anymore.
Additionally, kUserBlocking is quite appropriate, as the initial
compilation does block a whole section of a web app from execution.

R=clemensb@chromium.org

Bug: v8:11088
Cq-Include-Trybots: luci.v8.try:v8_linux64_tsan_rel_ng
Cq-Include-Trybots: luci.v8.try:v8_linux64_tsan_isolates_rel_ng
Change-Id: I6e1bcc809148198a4b4f88bfd4f2e62b1b061439
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2563675
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71546}