Expose the content of Maps and WeakMaps through MapMirror.

src/runtime.h

Index: src/runtime.h
diff --git a/src/runtime.h b/src/runtime.h
index fc561da17937a9f6d20e2b7e1e05baf72888500e..049f27909ebf2adecf748b0aa9bd0e10b48809ca 100644
--- a/src/runtime.h
+++ b/src/runtime.h
@@ -296,6 +296,8 @@ namespace internal {
   F(WeakCollectionDelete, 2, 1)                        \
   F(WeakCollectionSet, 3, 1)                           \
                                                        \
+  F(GetWeakMapEntries, 1, 1)                           \
+                                                       \
   /* Harmony events */                                 \
   F(EnqueueMicrotask, 1, 1)                            \
   F(RunMicrotasks, 0, 1)                               \