RFR (S): 8227084: Add timing information for merge heap root preparation

[Thomas Schatzl]

Hi all,

  can I have reviews for this change that adds timing information for
parts of the Merge Heap Roots phase as preparation for JDK-8226913.

CR:
https://bugs.openjdk.java.net/browse/JDK-8227084
Webrev:
http://cr.openjdk.java.net/~tschatzl/8227084
Testing:
hs-tier1-5

Thanks,
  Thomas