TimeToSafepoint is the time needed for all background threads to enter
a safepoint after the GC was started on the main thread. This CL
improves that metric during concurrent compilation to bytecode by doing:

1) Park the LocalIsolate during
   InterpreterCompilationJob::ExecuteJobImpl. There are no concurrent
   heap accesses happening while generating bytecode for now. So instead
   of manually placing Safepoint() invocations in the code, simply park
   the local isolate.
2) Destroy the LocalIsolate before the ReleaseParser operation. I've
   seen this take around 2ms, which regressed TimeToSafepoint a lot.
3) Add explicit safepoints to concurrent allocations. This covers the
   rest of the code and from what I've seen so far this is good enough
   to keep TimeToSafepoint around a few microseconds.

I've still seen TimeToSafepoint events with 20-80 microseconds but those
were quite rare and always seemed to be related to Turbofan.

AsLocalIsolate() is necessary in generic code to convert both Isolate
and LocalIsolate to LocalIsolate.

Bug: v8:10315
Change-Id: Idaf9f04ffdf850d0ab0081ec372cc384a9fe7ef9
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2663159Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72618}