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

tools/clang/blink_gc_plugin/BlinkGCPlugin.cpp

Index: tools/clang/blink_gc_plugin/BlinkGCPlugin.cpp
diff --git a/tools/clang/blink_gc_plugin/BlinkGCPlugin.cpp b/tools/clang/blink_gc_plugin/BlinkGCPlugin.cpp
index d263a8cc5eea20fa9bae021db02fe247ecf004b6..099b2bd96bdd4f2ad0fbaa71b28bd2fd26ea046f 100644
--- a/tools/clang/blink_gc_plugin/BlinkGCPlugin.cpp
+++ b/tools/clang/blink_gc_plugin/BlinkGCPlugin.cpp
@@ -33,6 +33,8 @@ class BlinkGCPluginAction : public PluginASTAction {
     for (const auto& arg : args) {
       if (arg == "dump-graph") {
         options_.dump_graph = true;
+      } else if (arg == "warn-stack-allocated-trace-method") {
+        options_.warn_stack_allocated_trace_method = true;
       } else if (arg == "warn-unneeded-finalizer") {
         options_.warn_unneeded_finalizer = true;
       } else if (arg == "use-chromium-style-naming") {