waiting_ flag is now set inside a lock to prevent data race. This means
that waiting_ is false when callback is called at start of wait. To deal
with the new behavior, NotifyWake now always tries to Notify and sets
interrupted_ flag which will be handled by any future wait.

R=binji@chromium.org
BUG=v8:8497

Change-Id: Ia4fd39bcf18875d9be21bafc176ab562b083e68b
Reviewed-on: https://chromium-review.googlesource.com/c/1351237
Commit-Queue: Aseem Garg <aseemgarg@chromium.org>
Reviewed-by: Ben Smith <binji@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57887}