Fix parent of the WeakMap prototype.

src/bootstrapper.cc

Index: src/bootstrapper.cc
diff --git a/src/bootstrapper.cc b/src/bootstrapper.cc
index f07e625ec01956ede51834e8616a1e4160e70c84..3ed334c1e208c73aba2bca03bcffa4ea981009b7 100644
--- a/src/bootstrapper.cc
+++ b/src/bootstrapper.cc
@@ -1204,10 +1204,9 @@ void Genesis::InitializeExperimentalGlobal() {
   // TODO(mstarzinger): Move this into Genesis::InitializeGlobal once we no
   // longer need to live behind a flag, so WeakMap gets added to the snapshot.
   if (FLAG_harmony_weakmaps) {  // -- W e a k M a p
-    Handle<JSObject> prototype =
-        factory()->NewJSObject(isolate()->object_function(), TENURED);
     InstallFunction(global, "WeakMap", JS_WEAK_MAP_TYPE, JSWeakMap::kSize,
-                    prototype, Builtins::kIllegal, true);
+                    isolate_->initial_object_prototype(),
+                    Builtins::kIllegal, true);
   }
 }