Initial idle GC logic.

runtime/vm/heap.h

Index: runtime/vm/heap.h
diff --git a/runtime/vm/heap.h b/runtime/vm/heap.h
index ad95cdab4498d1299df931b61f7321a99943a67a..759f3c37dfea3dc76cecaa285014917410d1aec7 100644
--- a/runtime/vm/heap.h
+++ b/runtime/vm/heap.h
@@ -47,6 +47,7 @@ class Heap {
     kPromotion,
     kOldSpace,
     kFull,
+    kIdle,
     kGCAtAlloc,
     kGCTestCase,
   };
@@ -104,6 +105,8 @@ class Heap {
   RawObject* FindNewObject(FindObjectVisitor* visitor) const;
   RawObject* FindObject(FindObjectVisitor* visitor) const;
 
+  void NotifyIdle(int64_t deadline);
+
   void CollectGarbage(Space space);
   void CollectGarbage(Space space, GCReason reason);
   void CollectAllGarbage();