Adds the possibility of setting a Code object as the callback of a FunctionTemplate.

src/ic/x64/handler-compiler-x64.cc

Index: src/ic/x64/handler-compiler-x64.cc
diff --git a/src/ic/x64/handler-compiler-x64.cc b/src/ic/x64/handler-compiler-x64.cc
index e48d3085ed7e5d63ebe7bea7ff7bff7efd5ad9cf..d7a3a4462aa1cdc24b66b38649b1d34d4a257824 100644
--- a/src/ic/x64/handler-compiler-x64.cc
+++ b/src/ic/x64/handler-compiler-x64.cc
@@ -188,6 +188,13 @@ void PropertyHandlerCompiler::GenerateApiAccessorCall(
     __ movp(data, FieldOperand(data, CallHandlerInfo::kDataOffset));
   }
 
+  if (api_call_info->fast_handler()->IsCode()) {
+    // Just tail call into the fast handler if present.
+    __ Jump(handle(Code::cast(api_call_info->fast_handler())),
+            RelocInfo::CODE_TARGET);
+    return;
+  }
+
   // Put api_function_address in place.
   Address function_address = v8::ToCData<Address>(api_call_info->callback());
   __ Move(api_function_address, function_address,