We have complicated logic in place to ensure that an {AsyncCompileJob}
is not removed as long as a foreground task is still pending.
This CL changes that to just cancel the pending foreground task and
remove the {AsyncCompileJob} immediately.

R=mstarzinger@chromium.org

Bug: chromium:869420
Change-Id: Ia064dae4a0e31416675e5d77c46879254fc817c4
Reviewed-on: https://chromium-review.googlesource.com/1158578Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54846}