Implement `in' for proxies.

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index fbb83159f16cd8b88d3454fdb21e71ec84d7f109..43747cc8fa1b6ffef0b698aaaf2c329f54399be3 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -1394,14 +1394,8 @@ class JSReceiver: public HeapObject {
   PropertyAttributes GetLocalPropertyAttribute(String* name);
 
   // Can cause a GC.
-  bool HasProperty(String* name) {
-    return GetPropertyAttribute(name) != ABSENT;
-  }
-
-  // Can cause a GC.
-  bool HasLocalProperty(String* name) {
-    return GetLocalPropertyAttribute(name) != ABSENT;
-  }
+  inline bool HasProperty(String* name);
+  inline bool HasLocalProperty(String* name);
 
   // Return the object's prototype (might be Heap::null_value()).
   inline Object* GetPrototype();
@@ -6481,6 +6475,8 @@ class JSProxy: public JSReceiver {
   // Casting.
   static inline JSProxy* cast(Object* obj);
 
+  bool HasPropertyWithHandler(String* name);
+
   MUST_USE_RESULT MaybeObject* SetPropertyWithHandler(
       String* name,
       Object* value,