8306321: Add an accessor for the top of a PLAB

Reviewed-by: shade, ysr

Author: William Kemper

src/hotspot/share/gc/shared/plab.hpp

@@ -138,6 +138,10 @@ class PLAB: public CHeapObj<mtGC> {
   // Fills in the unallocated portion of the buffer with a garbage object and updates
   // statistics. To be called during GC.
   void retire();
+
+  HeapWord* top() const {
+    return _top;
+  }
 };
 
 // PLAB book-keeping.