Fix windows build after JDK-8221507

The changes for JDK-8221507 broke the windows build:


shenandoahHeapRegion.cpp
.../jdk/src/hotspot/share/gc/shenandoah/shenandoahHeapRegion.cpp(687): error C2220: warning treated as error - no 'object' file generated
.../jdk/src/hotspot/share/gc/shenandoah/shenandoahHeapRegion.cpp(687): warning C4267: 'argument': conversion from 'size_t' to 'unsigned int', possible loss of data
   ... (rest of output omitted)
* For target hotspot_variant-server_libjvm_objs_shenandoahJfrSupport.obj:
shenandoahJfrSupport.cpp
.../jdk/src/hotspot/share/gc/shenandoah/shenandoahJfrSupport.cpp(60): error C2220: warning treated as error - no 'object' file generated
.../jdk/src/hotspot/share/gc/shenandoah/shenandoahJfrSupport.cpp(60): warning C4267: 'argument': conversion from 'size_t' to 'unsigned int', possible loss of data
   ... (rest of output omitted)