6794422: Perm gen expansion policy for concurrent collectors

Concurrent collectors should expand the perm gen without a full STW GC, but possibly by triggering a concurrent collection. Temporary band-aid for G1 where no concurrent collection is kicked off since the perm gen is not collected concurrently.

Reviewed-by: johnc

hotspot/src/share/vm/gc_implementation/concurrentMarkSweep/cmsPermGen.hpp

@@ -33,6 +33,10 @@ class CMSPermGen:  public PermGen {
   // The "generation" view.
   ConcurrentMarkSweepGeneration* _gen;
 
+  // Override default implementation from PermGen
+  virtual HeapWord* request_expand_and_allocate(Generation* gen, size_t size,
+                                                GCCause::Cause prev_cause);
+
  public:
   CMSPermGen(ReservedSpace rs, size_t initial_byte_size,
              CardTableRS* ct, FreeBlockDictionary::DictionaryChoice);