RFR: 8257509: Strengthen requirements to call G1HeapVerifier::verify(VerifyOption)

[Thomas Schatzl]

Hi all,

  can I have reviews for this little change to strengthen the requirements for calling G1HeapVerifier::verify(VerifyOption)?

In particular, instead of the failed attempt to abort verification if we are not at a safepoint, assert that we are at a safepoint.

Testing: tier1-5 with no failures

Thanks,
  Thomas

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Commit messages:
 - Initial import, testing

Changes: https://git.openjdk.java.net/jdk/pull/1590/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1590&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8257509
  Stats: 3 lines in 1 file changed: 0 ins; 2 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1590.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1590/head:pull/1590

PR: https://git.openjdk.java.net/jdk/pull/1590