At the moment all tasks which are spawned during asynchronous
compilation are CancelableTasks. However, we don't ever really cancel
tasks, and in the cases where we do it actually makes no sense.
Additionally, using CancelableTasks causes problems when V8 shuts down.
Therefore this CL switches to normal v8::Tasks instead of
CancelableTasks.

R=clemensh@chromium.org, mtrofin@chromium.org
BUG=v8:6253

Change-Id: Idf972fa042e2614a3b25faa4537416a772990bd3
Reviewed-on: https://chromium-review.googlesource.com/485760Reviewed-by: Mircea Trofin <mtrofin@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44855}