Ensure JSProxy correctness for PrototypeIterator uses

src/crankshaft/hydrogen.cc

Index: src/crankshaft/hydrogen.cc
diff --git a/src/crankshaft/hydrogen.cc b/src/crankshaft/hydrogen.cc
index 555291f588efac8062d74813d2be691a32455341..aa831515e0c1e569476f5dfe839f486fec0024e1 100644
--- a/src/crankshaft/hydrogen.cc
+++ b/src/crankshaft/hydrogen.cc
@@ -7375,6 +7375,8 @@ HInstruction* HOptimizedGraphBuilder::BuildMonomorphicElementAccess(
     PrototypeIterator iter(map);
     JSObject* holder = NULL;
     while (!iter.IsAtEnd()) {
+      // JSProxies can't occur here because we wouldn't have installed a
+      // non-generic IC if there were any.
       holder = *PrototypeIterator::GetCurrent<JSObject>(iter);
       iter.Advance();
     }