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

tools/clang/blink_gc_plugin/BlinkGCPluginOptions.h

Index: tools/clang/blink_gc_plugin/BlinkGCPluginOptions.h
diff --git a/tools/clang/blink_gc_plugin/BlinkGCPluginOptions.h b/tools/clang/blink_gc_plugin/BlinkGCPluginOptions.h
index baaa2fff6f14b087de46291bfb45075c974bdf9c..0df51cd597cc73c3f310ba54e6fb174a3f8a2a29 100644
--- a/tools/clang/blink_gc_plugin/BlinkGCPluginOptions.h
+++ b/tools/clang/blink_gc_plugin/BlinkGCPluginOptions.h
@@ -11,9 +11,17 @@
 
 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;