CompileTasks contain a pointer to the isolate. All tasks which have a
pointer to the isolate have to be cancelable to make sure that the task
does not execute after the isolate shut down.

R=rmcilroy@chromium.org

Change-Id: I10b2af0177b5cb60ab1dfdad47529fb8c7301ba0
Reviewed-on: https://chromium-review.googlesource.com/774441Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49418}