[heap] Use safer predicate for API functions.

This uses the common predicate SharedFunctionInfo::IsApiFunction to
determine flushability. Just a minor cleanup, no change in semantics.

R=hpayer@chromium.org

Review URL: https://codereview.chromium.org/1878183002

Cr-Commit-Position: refs/heads/master@{#35419}

src/heap/objects-visiting-inl.h

@@ -633,8 +633,7 @@ bool StaticMarkingVisitor<StaticVisitor>::IsFlushable(
   }
 
   // We never flush code for API functions.
-  Object* function_data = shared_info->function_data();
-  if (function_data->IsFunctionTemplateInfo()) {
+  if (shared_info->IsApiFunction()) {
     return false;
   }