There exists a hidden assumption in V8 that neither foreground nor
background tasks own any memory. For asynchronous WebAssembly
compilation this assumption was wrong, which causes crashes when V8 shut
down before the compilation finished.

With this CL I change the way asynchrous compilation happens. In the
existing implementation each compilation stage provided its own task
which could be spawned either in foreground or background. With this CL
each stage only provides a state, and a generic CompileTask executes on
that state. There exists exactly one state at a time.

To have exactly one state at a time I combined the stages
ExecuteCompilationUnits and FinishCompilationUnits to a single stage. In
addition I removed the WaitForBackgroundTasks stage and added a
CancelableTaskManager to the AsyncCompileJob instead to do the waiting.

BUG=v8:6436
R=clemensh@chromium.org, mtrofin@chromium.org

Change-Id: I2eb61f74235c65524ce720c474eaf99ae7472c81
Reviewed-on: https://chromium-review.googlesource.com/532993
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45908}