RFR: JDK-8145303: Clean up the units for log_gc_footer

[Bengt Rutisson]

Hi everyone,

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

http://cr.openjdk.java.net/~brutisso/8145303/webrev.00/
https://bugs.openjdk.java.net/browse/JDK-8145303

The log_gc_footer() method takes one parameter of type double. But it 
should be jlong. It also converts to seconds but farther down the call 
chain we convert to milliseconds.

Thanks,
Bengt