7099824: G1: we should take the pending list lock before doing the remark pause Summary: Acquire the pending list lock in the prologue method of G1's concurrent VM_Operation and release the lock in the epilogue() method. The locking/unlocking order of the pending list lock and the Heap_lock should match that in the prologue and epilogue methods of VM_GC_Operation. Reviewed-by: tonyp, ysr