We should only cancel asynchronous compilation jobs for the isolate
which is being recycled.

R=titzer@chromium.org

Bug: chromium:854755
Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: I31d6c3ccb648f5465e52f4bc47c4261894458e60
Reviewed-on: https://chromium-review.googlesource.com/1118378Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54092}