Add HeapTest and make GC work enough to pass it

Change-Id: If06eaef2921b64b3226bfd347acaec60ec993e67
diff --git a/src/runtime.h b/src/runtime.h
index 8694b3c..40db814 100644
--- a/src/runtime.h
+++ b/src/runtime.h
@@ -12,6 +12,7 @@
 
 #include <jni.h>
 
+#include "heap.h"
 #include "globals.h"
 #include "macros.h"
 #include "stringpiece.h"
@@ -93,6 +94,8 @@
     return java_vm_;
   }
 
+  void VisitRoots(Heap::RootVistor* root_visitor, void* arg) const;
+
  private:
   static void PlatformAbort(const char*, int);