RFR: 8293515: heapShared.cpp: rename JavaThread parameter to current

[Matias Saavedra Silva]

Replaced instances of THREAD with current and placed the parameter at the beginning rather than the end. Necessary instances of THREAD were initialized with current. Verified with tier 1 tests. Please review.

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Commit messages:
 - 8293515: heapShared.cpp: rename JavaThread parameter to current

Changes: https://git.openjdk.org/jdk/pull/10451/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10451&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8293515
  Stats: 17 lines in 3 files changed: 2 ins; 2 del; 13 mod
  Patch: https://git.openjdk.org/jdk/pull/10451.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10451/head:pull/10451

PR: https://git.openjdk.org/jdk/pull/10451