RFR: 8209163: SA: Show Object Histogram asserts with ZGC

[Per Liden]

This patch avoids an assertion, and instead prints a warning, when 
trying to show the "Object Histogram" when using ZGC. I also had to add 
a call to heapIterationFractionUpdate() so that the update progress 
frame is properly created, even when there are not heap regions to walk 
over. Without this, you only get a call to heapIterationCompleted(), 
with a null frame, which throws a NullPointerException. Always making a 
call to heapIterationFractionUpdate(0.0) in the prologue avoids this by 
making sure we always created the frame, even if no more fraction 
updates will happen.

Bug: https://bugs.openjdk.java.net/browse/JDK-8209163
Webrev: http://cr.openjdk.java.net/~pliden/8209163/webrev.0

/Per