RFR: 8278276: G1: Refine naming of G1GCParPhaseTimesTracker::_must_record

[Hamlin Li]

This is a trivial improvement to refine the naming of G1GCParPhaseTimesTracker::_must_record to _initial_record.

-------------

Commit messages:
 - Initial commit

Changes: https://git.openjdk.java.net/jdk/pull/6714/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6714&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8278276
  Stats: 6 lines in 2 files changed: 0 ins; 0 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6714.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6714/head:pull/6714

PR: https://git.openjdk.java.net/jdk/pull/6714