Reland "Enable Object.observe by default" again

src/accessors.cc

Index: src/accessors.cc
diff --git a/src/accessors.cc b/src/accessors.cc
index ac64683d091341dbab2628bba4383bc4d6878f1e..99ea25ad7f56433c161c5ba8869122cda42cbe00 100644
--- a/src/accessors.cc
+++ b/src/accessors.cc
@@ -600,10 +600,7 @@ MaybeObject* Accessors::FunctionSetPrototype(Isolate* isolate,
   }
 
   Handle<Object> old_value;
-  bool is_observed =
-      FLAG_harmony_observation &&
-      *function == *object &&
-      function->map()->is_observed();
+  bool is_observed = *function == *object && function->map()->is_observed();
   if (is_observed) {
     if (function->has_prototype())
       old_value = handle(function->prototype(), isolate);