The {priority_} field is being updated in {DefaultJobState::Join}, under
{mutex_}. In other places though, it is read unprotected (without
holding the mutex), leading to data races.
This CL fixes that by reading the field while holding the mutex and
using the read priority after releasing the mutex.

Note that the {priority_} field is documented to be protected by
{mutex_}, so the unprotected read was a bug.

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

Bug: v8:10822
Change-Id: I80079f3cb6689e26116ffeb33755c6938c4a2cf1
Cq-Include-Trybots: luci.v8.try:v8_linux64_tsan_rel_ng
Cq-Include-Trybots: luci.v8.try:v8_linux64_tsan_isolates_rel_ng
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2377685Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69571}