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

tools/clang/blink_gc_plugin/tests/stack_allocated_no_warning.h

Index: tools/clang/blink_gc_plugin/tests/stack_allocated_no_warning.h
diff --git a/tools/clang/blink_gc_plugin/tests/stack_allocated_no_warning.h b/tools/clang/blink_gc_plugin/tests/stack_allocated_no_warning.h
new file mode 100644
index 0000000000000000000000000000000000000000..fe2c95ab36359ca5df738754bf6c36d0ae74ccac
--- /dev/null
+++ b/tools/clang/blink_gc_plugin/tests/stack_allocated_no_warning.h
@@ -0,0 +1,26 @@
+// Copyright 2017 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 STACK_ALLOCATED_H_
+#define STACK_ALLOCATED_H_
+
+#include "heap/stubs.h"
+
+namespace blink {
+
+class HeapObject;
+
+class StackObject {
+    STACK_ALLOCATED();
+
+    // Redundant trace() method, but warning/error disabled.
+    void Trace(Visitor* visitor) { visitor->Trace(m_obj); }
+
+private:
+    Member<HeapObject> m_obj; // Does not need tracing.
+};
+
+}
+
+#endif