[turbofan] Call FastNewContextStub for function context.

src/code-factory.h

Index: src/code-factory.h
diff --git a/src/code-factory.h b/src/code-factory.h
index 5a67b27fd54b07caa76ea9fd5d3229a3761a6140..97dbcccc40299394d54120367f8a4bb353fe7208 100644
--- a/src/code-factory.h
+++ b/src/code-factory.h
@@ -85,6 +85,7 @@ class CodeFactory final {
   static Callable FastCloneShallowArray(Isolate* isolate);
   static Callable FastCloneShallowObject(Isolate* isolate, int length);
 
+  static Callable FastNewContext(Isolate* isolate, int slot_count);
   static Callable FastNewClosure(Isolate* isolate, LanguageMode language_mode,
                                  FunctionKind kind);