This CL makes foreground compilation tasks normal (i.e. not cancelable)
again, because otherwise a deadlock can happen. I think the reason why
the foreground tasks were cancelable was to make sure that all tasks
either finish correctly or get canceled. However, since the isolate can
only shut down on the main thread, this means that the foreground task
should have already finished when the isolate shuts down, or it should
not have started at all. I reordered the deletion of the AsyncCompileJob
though to make sure that an AsyncCompileJob is removed from
CompilationManager before its promise is resolved.

Here is the deadlock: The JS code which is executed after a promise is
resolved is executed within the task which resolves the promise. In case
of async compilation this means that some JS code is executed within a
CompileTask. In JS, the shutdown of the isolate can be triggered. During
the shutdown of the isolate, the CancelableTaskManager waits for all
registered cancelable tasks to complete, including the CompileTask of
async compilation. This means that the CancelableTaskManager waits for
itself to finish, which is a deadlock.

R=clemensh@chromium.org, mtrofin@chromium.org

Change-Id: I9f8c7fb2cfc5b9bfc53c761010b1590293bb82c9
Reviewed-on: https://chromium-review.googlesource.com/554733
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Mircea Trofin <mtrofin@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46343}