RFR(XS): 7129514: time warp warnings after 7117303

[John Cuthbertson]

Hi Everyone,

While making the changes for 711303, I missed a few calls to 
os::javaTimeMillis() (specifically with updating the time since the last 
GC). As a consequence we can still see the occasional time-warp warning. 
The issue is that os::javaTimeMillis() returns values that are not 
guaranteed to be monotonically non-decreasing and so they can go 
backwards.  I've replaced these calls to an equivalent that uses 
os::javaTimeNanos(), which will return values that are monotonically 
non-decreasing if the underlying system time source supports such a mode.

Many thanks to David Holmes for diagnosing the issue.

Thanks,

JohnC