GCController::garbageCollectNowIfNotDoneRecently should request Async Full GCs

GCController::garbageCollectNowIfNotDoneRecently should request Async Full GCs
https://bugs.webkit.org/show_bug.cgi?id=172204

Reviewed by Saam Barati.

No new tests because existing tests will tell us if there is a problem.

The goal of this change is to reduce the likelihood that we block for a GC. We want it to be
benchmark-neutral.

It's a 0.14% speed-up on JetStream with 24% probability.

It's a 0.12% slow-down on PLT3 with 43% probability.

So it's neutral on my machine.

* bindings/js/GCController.cpp:
(WebCore::GCController::garbageCollectNowIfNotDoneRecently):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@216965 268f45cc-cd09-0410-ab3c-d52691b4dbfc