Background compile jobs should not keep the NativeModule alive, for two
reasons:
1) We sometimes have to wait for background compilation to finish (from
   a foreground task!). This introduces unnecessary latency.
2) Giving the background compile tasks shared ownership of the
   NativeModule causes the NativeModule (and the CompilationState) to
   be freed from background tasks, which is error-prone (see
   https://crrev.com/c/1400420).

Instead, this CL introduces a BackgroundCompileToken which is held
alive by the NativeModule and all background compile jobs. The initial
and the final phase of compilation (getting and submitting work)
synchronize on this token to check and ensure that the NativeModule is
and stays alive. During compilation itself, the mutex is released, such
that the NativeModule can die.
The destructor of the NativeModule cancels the BackgroundCompileToken.
Immediately afterwards, the NativeModule and the CompilationState can
die.

This change allows to remove two hacks introduced previously: The atomic
{aborted_} flag and the {FreeCallbacksTask}.

R=mstarzinger@chromium.org
CC=titzer@chromium.org

Bug: v8:8689, v8:7921
Change-Id: I42e06eab3c944b0988286f2ce18e3c294535dfb6
Cq-Include-Trybots: luci.v8.try:v8_linux64_tsan_rel
Reviewed-on: https://chromium-review.googlesource.com/c/1421364
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59020}