It's too easy to implicitly cast it to bool, as we did in several tests.
Also, move TryAbortResult out of CancelableTaskManager to avoid too much
typing when referencing one of the enum values.

R=mstarzinger@chromium.org

Bug: v8:8238
Change-Id: Ia3fa8597428876217bc86f9b8b31c21ae4846fa1
Reviewed-on: https://chromium-review.googlesource.com/c/1326027
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57363}