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 c8aa3996665b595978f6657565576af82e426a6b..a82a9c7e562ada662aa90fbb11ad00cee1e973e1 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();