RFR: 8297285: Shenandoah pacing causes assertion failure during VM initialization

[Ashutosh Mehra]

Please review the fix for the assertion failure seen during VM init due to pacing in shenandoah gc.
The fix is to avoid pacing during VM initialization as the main thread is not yet an active java thread.

Signed-off-by: Ashutosh Mehra <asmehra at redhat.com>

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Commit messages:
 - 8297285: Shenandoah pacing causes assertion failure during VM

Changes: https://git.openjdk.org/jdk/pull/11360/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11360&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8297285
  Stats: 7 lines in 1 file changed: 6 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/11360.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11360/head:pull/11360

PR: https://git.openjdk.org/jdk/pull/11360