RFR: 8080877: Don't use workers()->total_workers() when walking G1CollectedHeap::_task_queues

[Stefan Karlsson]

Hi all,

The _task_queues are initialized to contain ParallelGCThreads number of 
queues:

   int n_queues = (int)ParallelGCThreads;
   _task_queues = new RefToScanQueueSet(n_queues);

When iterating over the queues we use workers()->total_workers() to 
determine the number of queues in the set:

   const uint n = workers()->total_workers();
   for (uint i = 0; i < n; ++i) {
     task_queue(i)->stats.reset();
   }

which gives the same result, since the workers are also initialized with 
ParallelGCThreads:

   _workers = new FlexibleWorkGang("GC Thread", ParallelGCThreads,
                           /* are_GC_task_threads */true,
                           /* are_ConcurrentGC_threads */false);
   _workers->initialize_workers();

I propose that we change the iteration code to ask the task queue set 
how many task queues it contain:

-  const uint n = workers()->total_workers();
+  const uint n = num_task_queues();
    for (uint i = 0; i < n; ++i) {
      task_queue(i)->stats.reset();
    }

I also removed the unnecessary forward declarations of OopTaskQueue and 
OopTaskQueueSet. This isn't strictly needed for this patch, but was 
causing problems while I was prototyping this and other patches.

http://cr.openjdk.java.net/~stefank/8080877/webrev.00/
https://bugs.openjdk.java.net/browse/JDK-8080877

Thanks,
StefanK