IsRunning is the v8 equivalent of operator bool, but is confusing
with IsCompleted. IsValid (to match base:: operator bool) should be more
clear.

Change-Id: I2529bea21c7cb7613bd5057c66715fb5ea450396
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2461840Reviewed-by: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70625}