Fix parent of the WeakMap prototype.

src/weakmap.js

Index: src/weakmap.js
diff --git a/src/weakmap.js b/src/weakmap.js
index 6c15e30fb6f3b6dfb82d565e1293818f3221165a..5fb5151071c2cd124ec75622d5eb91708fa22253 100644
--- a/src/weakmap.js
+++ b/src/weakmap.js
@@ -85,9 +85,6 @@ function WeakMapDelete(key) {
   // Set up the WeakMap constructor function.
   %SetCode($WeakMap, WeakMapConstructor);
 
-  // Set up the WeakMap prototype object.
-  %FunctionSetPrototype($WeakMap, new $WeakMap());
-
   // Set up the constructor property on the WeakMap prototype object.
   %SetProperty($WeakMap.prototype, "constructor", $WeakMap, DONT_ENUM);