Support StringLength in hydrogen (similar to ArrayLength).

src/ast.cc

Index: src/ast.cc
diff --git a/src/ast.cc b/src/ast.cc
index e8b3e034509c4cba20f40d360cbf0f07f3b2996e..11cf334f933160be7a49a382a74dd1efc3a3210c 100644
--- a/src/ast.cc
+++ b/src/ast.cc
@@ -521,6 +521,8 @@ void Property::RecordTypeFeedback(TypeFeedbackOracle* oracle) {
   if (key()->IsPropertyName()) {
     if (oracle->LoadIsBuiltin(this, Builtins::LoadIC_ArrayLength)) {
       is_array_length_ = true;
+    } else if (oracle->LoadIsBuiltin(this, Builtins::LoadIC_StringLength)) {
+      is_string_length_ = true;
     } else if (oracle->LoadIsBuiltin(this,
                                      Builtins::LoadIC_FunctionPrototype)) {
       is_function_prototype_ = true;