RFR: 8264682: MemProfiling does not own Heap_lock when using G1

[Yi Yang]

Trivial fix for JDK-8264682.

`Universe::heap()->used()` calls G1Allocator::used_in_alloc_regions() when G1 enabled,  it checks whether Heap_lock was owned on this thread's behalf.

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Commit messages:
 - memprofiling_crash

Changes: https://git.openjdk.java.net/jdk/pull/3340/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3340&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8264682
  Stats: 59 lines in 2 files changed: 59 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3340.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3340/head:pull/3340

PR: https://git.openjdk.java.net/jdk/pull/3340