Eliminate special keyed load string stub in favor of uniform handlers.

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index 0e8d2b460cb263f2896b4fb89007e16d5b94e551..f16ca9e566678c1ee259bd0a173d43d69d20b575 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -6124,6 +6124,7 @@ class Map: public HeapObject {
   bool IsJSObjectMap() {
     return instance_type() >= FIRST_JS_OBJECT_TYPE;
   }
+  bool IsStringMap() { return instance_type() < FIRST_NONSTRING_TYPE; }
   bool IsJSProxyMap() {
     InstanceType type = instance_type();
     return FIRST_JS_PROXY_TYPE <= type && type <= LAST_JS_PROXY_TYPE;