Revert "[Object.observe] Don't force normalization of elements for observed objects"

src/objects-debug.cc

Index: src/objects-debug.cc
diff --git a/src/objects-debug.cc b/src/objects-debug.cc
index 80610c6a8b5de272099591d1bff2b688be818d5d..6ab2ddffe2a9e0ea3772163e20e87bd269f1a7e0 100644
--- a/src/objects-debug.cc
+++ b/src/objects-debug.cc
@@ -366,6 +366,9 @@ void Map::MapVerify() {
     SLOW_ASSERT(transitions()->IsSortedNoDuplicates());
     SLOW_ASSERT(transitions()->IsConsistentWithBackPointers(this));
   }
+  ASSERT(!is_observed() || instance_type() < FIRST_JS_OBJECT_TYPE ||
+         instance_type() > LAST_JS_OBJECT_TYPE ||
+         has_slow_elements_kind() || has_external_array_elements());
 }