Modernize unittest for cancelable tasks

1) Use own test fixture {CancelableTaskManagerTest}.
2) Avoid base::AtomicWord.
3) Use unique_ptr.
4) Fix order of expected and actual values in EXPECT_EQ.

R=mstarzinger@chromium.org

Bug: v8:8238
Change-Id: I3d29785864bbf3de58c5d9d5384b9e0065255e99
Reviewed-on: https://chromium-review.googlesource.com/c/1325967
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57378}