RFR (XXS): 8210463: Recalculate_used() always sets time taken in G1GCPhaseTimes

[Thomas Schatzl]

Hi,

  can I have reviews for this refactoring of
G1CollectedHeap::recalculate_used() to move out time measurement from
this method to the only place where we intend to measure the time that
operation takes?
In the other cases we use recalculate_used() only in asserts.

While it does not really hurt because the wrong values are never read
from, it's still a matter of cleaning up the code imho.

CR:
https://bugs.openjdk.java.net/browse/JDK-8210463
Webrev:
http://cr.openjdk.java.net/~tschatzl/8210463/webrev/
Testing:
manual checking that time is still reported for the only case we care

Thanks,
  Thomas