Implement for-in loop for proxies.

src/contexts.h

Index: src/contexts.h
diff --git a/src/contexts.h b/src/contexts.h
index b80475f0f7c4e199e1544eabed91920a60cdd65f..301e6dd3be75866b52227c1073ce875d5c964931 100644
--- a/src/contexts.h
+++ b/src/contexts.h
@@ -138,7 +138,8 @@ enum BindingFlags {
     to_complete_property_descriptor) \
   V(DERIVED_HAS_TRAP_INDEX, JSFunction, derived_has_trap) \
   V(DERIVED_GET_TRAP_INDEX, JSFunction, derived_get_trap) \
-  V(DERIVED_SET_TRAP_INDEX, JSFunction, derived_set_trap)
+  V(DERIVED_SET_TRAP_INDEX, JSFunction, derived_set_trap) \
+  V(PROXY_ENUMERATE, JSFunction, proxy_enumerate)
 
 // JSFunctions are pairs (context, function code), sometimes also called
 // closures. A Context object is used to represent function contexts and
@@ -258,6 +259,7 @@ class Context: public FixedArray {
     DERIVED_HAS_TRAP_INDEX,
     DERIVED_GET_TRAP_INDEX,
     DERIVED_SET_TRAP_INDEX,
+    PROXY_ENUMERATE,
 
     // Properties from here are treated as weak references by the full GC.
     // Scavenge treats them as strong references.