RFR (S): 8245086: G1: Rename measured pause time ratios

[Thomas Schatzl]

Hi all,

   can I have reviews for this small change that improves the 
readability of the heap sizing code quite a bit and offers more 
flexibility in the future in how the long and short term pause time 
ratios are defined?

This is not about *fixing* the calculation mentioned in JDK-8243672, 
just renames.

CR:
https://bugs.openjdk.java.net/browse/JDK-8245086
Webrev:
http://cr.openjdk.java.net/~tschatzl/8245086/
Testing:
hs-tier1-5

Thanks,
   Thomas