When resetting the {unique_ptr} to the {CompilationState} in the
{NativeModule}, what actually happens is that first the pointer stored
in the {unique_ptr} is reset to {nullptr}, then the destructor is
called.
The destructor of {CompilationState} cancels and waits for background
compile jobs. While doing so, background compile jobs still try to
access the {unique_ptr} in the {NativeModule}.

This CL fixes this race by splitting the shutdown in two steps: First,
cancel and wait the background compile jobs, and only later reset the
pointer.

R=ahaas@chromium.org

Bug: v8:8359
No-Tree-Checks: true
Change-Id: Ifa3bdf3424dfd5a4712d33f8ca85f9382b1766a6
Reviewed-on: https://chromium-review.googlesource.com/c/1296486
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56913}