KeyedStoreIC: support non-generic handling of Symbol keys

BUG=chromium:450475
LOG=n

Review URL: https://codereview.chromium.org/856333002

Cr-Commit-Position: refs/heads/master@{#26188}

src/ic/ic.cc

@@ -2035,10 +2035,10 @@ MaybeHandle<Object> KeyedStoreIC::Store(Handle<Object> object,
   Handle<Object> store_handle;
   Handle<Code> stub = megamorphic_stub();
 
-  if (key->IsInternalizedString()) {
+  if (key->IsInternalizedString() || key->IsSymbol()) {
     ASSIGN_RETURN_ON_EXCEPTION(
         isolate(), store_handle,
-        StoreIC::Store(object, Handle<String>::cast(key), value,
+        StoreIC::Store(object, Handle<Name>::cast(key), value,
                        JSReceiver::MAY_BE_STORE_FROM_KEYED),
         Object);
     if (!is_target_set()) {