Make FastNewFunctionContextStub take slots parameter

src/x64/interface-descriptors-x64.cc

Index: src/x64/interface-descriptors-x64.cc
diff --git a/src/x64/interface-descriptors-x64.cc b/src/x64/interface-descriptors-x64.cc
index 6caf7c5f082033ff5d8a80b79f624be3c6b9df56..b6040dfc09fadab513dd9a0a92fb769235a26577 100644
--- a/src/x64/interface-descriptors-x64.cc
+++ b/src/x64/interface-descriptors-x64.cc
@@ -20,6 +20,11 @@ void CallInterfaceDescriptor::DefaultInitializePlatformSpecific(
                                    default_stub_registers);
 }
 
+const Register FastNewFunctionContextDescriptor::FunctionRegister() {
+  return rdi;
+}
+const Register FastNewFunctionContextDescriptor::SlotsRegister() { return rax; }
+
 const Register LoadDescriptor::ReceiverRegister() { return rdx; }
 const Register LoadDescriptor::NameRegister() { return rcx; }
 const Register LoadDescriptor::SlotRegister() { return rax; }
@@ -71,12 +76,6 @@ void FastNewClosureDescriptor::InitializePlatformSpecific(
   data->InitializePlatformSpecific(arraysize(registers), registers);
 }
 
-void FastNewFunctionContextDescriptor::InitializePlatformSpecific(
-    CallInterfaceDescriptorData* data) {
-  Register registers[] = {rdi};
-  data->InitializePlatformSpecific(arraysize(registers), registers);
-}
-
 void FastNewObjectDescriptor::InitializePlatformSpecific(
     CallInterfaceDescriptorData* data) {
   Register registers[] = {rdi, rdx};