[runtime] Introduce dedicated JSBoundFunction to represent bound functions.

src/api.cc

Index: src/api.cc
diff --git a/src/api.cc b/src/api.cc
index 93ea6a4c028be4b3a9d540be5ab8c9ad8ce018a2..10afa2c43f3731fdba7140effad9051b564175e5 100644
--- a/src/api.cc
+++ b/src/api.cc
@@ -4537,18 +4537,13 @@ int Function::ScriptId() const {
 
 Local<v8::Value> Function::GetBoundFunction() const {
   auto self = Utils::OpenHandle(this);
-  if (!self->IsJSFunction()) {
-    return v8::Undefined(reinterpret_cast<v8::Isolate*>(self->GetIsolate()));
+  if (self->IsJSBoundFunction()) {
+    auto bound_function = i::Handle<i::JSBoundFunction>::cast(self);
+    auto bound_target_function = i::handle(
+        bound_function->bound_target_function(), bound_function->GetIsolate());
+    return Utils::CallableToLocal(bound_target_function);
   }
-  auto func = i::Handle<i::JSFunction>::cast(self);
-  if (!func->shared()->bound()) {
-    return v8::Undefined(reinterpret_cast<v8::Isolate*>(func->GetIsolate()));
-  }
-  i::Handle<i::BindingsArray> bound_args = i::Handle<i::BindingsArray>(
-      i::BindingsArray::cast(func->function_bindings()));
-  i::Handle<i::Object> original(bound_args->bound_function(),
-                                func->GetIsolate());
-  return Utils::CallableToLocal(i::Handle<i::JSFunction>::cast(original));
+  return v8::Undefined(reinterpret_cast<v8::Isolate*>(self->GetIsolate()));
 }