Implement for-in loop for proxies.

src/contexts.h

Index: src/contexts.h
diff --git a/src/contexts.h b/src/contexts.h
index db725c36d9d85d37032b1990e0bafd95079a7d99..c30fe6e9a9e00a6b30634f425370411b79c54205 100644
--- a/src/contexts.h
+++ b/src/contexts.h
@@ -139,6 +139,7 @@ enum BindingFlags {
   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(PROXY_ENUMERATE, JSFunction, proxy_enumerate) \
   V(RANDOM_SEED_INDEX, ByteArray, random_seed)
 
 // JSFunctions are pairs (context, function code), sometimes also called
@@ -260,6 +261,7 @@ class Context: public FixedArray {
     DERIVED_HAS_TRAP_INDEX,
     DERIVED_GET_TRAP_INDEX,
     DERIVED_SET_TRAP_INDEX,
+    PROXY_ENUMERATE,
     RANDOM_SEED_INDEX,
 
     // Properties from here are treated as weak references by the full GC.