Flip -XX:+UseShenandoahGC on by default in shenandoah/jdk9?

[Aleksey Shipilev]

Hi,

I wonder if we want to flip -XX:+UseShenandoahGC on by default in our
forest. This will allow us to reuse all existing GC-agnostic Hotspot
tests for testing Shenandoah -- I'm looking at compiler tests mostly.

Thanks,
-Aleksey