reland 16744: add context save for GenerateFastApiCall

include/v8.h

Index: include/v8.h
diff --git a/include/v8.h b/include/v8.h
index dceebebb698315c7ad44145ac0ea7c44c35ed7ac..92bfa45e6ff99def943b80b8fc3159b01ee0a207 100644
--- a/include/v8.h
+++ b/include/v8.h
@@ -2364,7 +2364,7 @@ class FunctionCallbackInfo {
   V8_INLINE Isolate* GetIsolate() const;
   V8_INLINE ReturnValue<T> GetReturnValue() const;
   // This shouldn't be public, but the arm compiler needs it.
-  static const int kArgsLength = 6;
+  static const int kArgsLength = 7;
 
  protected:
   friend class internal::FunctionCallbackArguments;
@@ -2375,6 +2375,7 @@ class FunctionCallbackInfo {
   static const int kDataIndex = -3;
   static const int kCalleeIndex = -4;
   static const int kHolderIndex = -5;
+  static const int kContextSaveIndex = -6;
 
   V8_INLINE FunctionCallbackInfo(internal::Object** implicit_args,
                    internal::Object** values,