RFR: 9+8u: ShenandoahHeap::_cancelled_gc should be consistently treated as jbyte

[Aleksey Shipilev]

Hi,

Compilers do complain when you use jbyte and bools interchangeably. Also we cast
out constness in setters, what-what? MSVC fails Windows build.

Fix:
  http://cr.openjdk.java.net/~shade/shenandoah/build/cancel-gc-jbyte/webrev.01/

This affects both 9 and 8u.

Testing: hotspot_gc_shenandoah, 9 and 8u

Thanks,
-Aleksey