RFR(xs): 8220151: SafepointTracing::end_of_last_safepoint_ms should return ms since epoch.

[Robbin Ehn]

Hi all, please review.

os::javaTimeNanos and os::javaTimeMillis may use different starting points.
nanos cannot be converted to millies when comparing to absolute ms time.
SafepointTracing::end_of_last_safepoint_ms must use os::javaTimeMillis.

Code:
http://cr.openjdk.java.net/~rehn/8220151/webrev/
Issue:
https://bugs.openjdk.java.net/browse/JDK-8220151

Passes t1.

Thanks, Robbin