RFR: 8280391: NMT: Correct NMT tag on CollectedHeap

[Zhengyu Gu]

Please review this trivial patch.

Heap is the centerpiece of GC, it is odd to see that it is tagged as mtInternal.

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Commit messages:
 - 8280391: NMT: Correct NMT tag on CollectedHeap

Changes: https://git.openjdk.java.net/jdk/pull/7161/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7161&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8280391
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7161.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7161/head:pull/7161

PR: https://git.openjdk.java.net/jdk/pull/7161