Reland of [keys] Simplify KeyAccumulator

src/runtime/runtime-forin.cc

Index: src/runtime/runtime-forin.cc
diff --git a/src/runtime/runtime-forin.cc b/src/runtime/runtime-forin.cc
index 1799b339e6b1b5c07406136dd29f6fb1af358ef0..6e73e36593f3943fca6c80392021f717f4628a93 100644
--- a/src/runtime/runtime-forin.cc
+++ b/src/runtime/runtime-forin.cc
@@ -25,6 +25,7 @@ MaybeHandle<HeapObject> Enumerate(Handle<JSReceiver> receiver) {
   FastKeyAccumulator accumulator(isolate, receiver, INCLUDE_PROTOS,
                                  ENUMERABLE_STRINGS);
   accumulator.set_filter_proxy_keys(false);
+  accumulator.set_is_for_in(true);
   // Test if we have an enum cache for {receiver}.
   if (!accumulator.is_receiver_simple_enum()) {
     Handle<FixedArray> keys;