Add assertion to UniqueValueId constructor.

src/heap.h

Index: src/heap.h
diff --git a/src/heap.h b/src/heap.h
index 4dfa076ebd7745c14e953cbf0deb3bd396eb5c5a..8bd3b0d7f0943181e7a980787f47dae6ca621ac3 100644
--- a/src/heap.h
+++ b/src/heap.h
@@ -1414,6 +1414,12 @@ class Heap {
   // Generated code can embed this address to get access to the roots.
   Object** roots_array_start() { return roots_; }
 
+#ifdef DEBUG
+  bool IsInRootsArray(Object** handle) {
+    return roots_ <= handle && handle < roots_ + Heap::kStrongRootListLength;
+  }
+#endif
+
   Address* store_buffer_top_address() {
     return reinterpret_cast<Address*>(&roots_[kStoreBufferTopRootIndex]);
   }