RFR (S): JDK-8079561: Add a method to convert coutners to milliseconds

[Bengt Rutisson]

Hi everyone,

Could I have a couple of reviews for this small change?

https://bugs.openjdk.java.net/browse/JDK-8079561
http://cr.openjdk.java.net/~brutisso/8079561/webrev.00/

TimeHelper contains a method called counter_to_seconds(jlong counter) 
that converts a counter, as returned by for example 
os:elapsed_counter(), into seconds. I'm currently working on a patch 
where I need to convert into milliseconds instead. So, I would like to 
add the method TimeHelper::counter_to_millis(jlong counter).

It turns out that there are already a few places where we convert to 
milliseconds. These can now use the new method instead of explicitly 
multiplying with 1000.

Thanks,
Bengt