blink_gc_plugin: warn of unused trace methods to stack allocated classes.

tools/clang/blink_gc_plugin/BlinkGCPluginOptions.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef TOOLS_BLINK_GC_PLUGIN_BLINK_GC_PLUGIN_OPTIONS_H_
 #define TOOLS_BLINK_GC_PLUGIN_BLINK_GC_PLUGIN_OPTIONS_H_
 
 #include <set>
 #include <string>
 #include <vector>
 
 struct BlinkGCPluginOptions {
   bool dump_graph = false;
+
+  // If |true|, emit warning if stack allocated class types provide
+  // a trace method.
+  bool warn_stack_allocated_trace_method = false;
+  // If |true|, emit warning for class types which derive from from
+  // GarbageCollectedFinalized<> when just GarbageCollected<> will do.
   bool warn_unneeded_finalizer = false;
+
   // TODO(https://crbug.com/675879): Clean up after the Blink rename.
   bool use_chromium_style_naming = false;
+
   std::set<std::string> ignored_classes;
   std::set<std::string> checked_namespaces;
   std::vector<std::string> ignored_directories;
 };
 
 #endif  // TOOLS_BLINK_GC_PLUGIN_BLINK_GC_PLUGIN_OPTIONS_H_