If we create a second foreground task, only the second one will be
registered with the AsyncCompileJob, so the first one will not be
cancelled, which can lead to use-after-free of the AsyncCompileJob.
In a debug build, a DCHECK will fail when creating the second
foreground task.

R=ahaas@chromium.org

Bug: chromium:907937, chromium:910920
Change-Id: Iefefc4a85e7b35b32051cfe8cd5cbbfc4e95b843
Reviewed-on: https://chromium-review.googlesource.com/c/1367684
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58108}