BlinkGCPlugin: require GC mixin instances to declare local trace().

tools/clang/blink_gc_plugin/tests/trace_templated_super.h

Index: tools/clang/blink_gc_plugin/tests/trace_templated_super.h
diff --git a/tools/clang/blink_gc_plugin/tests/trace_templated_super.h b/tools/clang/blink_gc_plugin/tests/trace_templated_super.h
index 09e52570c6fe2c7efd4a8e26df4d010f0877abe8..de8fd7b4c58d1e73b485cfefd23db643d459c835 100644
--- a/tools/clang/blink_gc_plugin/tests/trace_templated_super.h
+++ b/tools/clang/blink_gc_plugin/tests/trace_templated_super.h
@@ -13,14 +13,14 @@ class HeapObject;
 
 class Mixin : public GarbageCollectedMixin {
 public:
-    virtual void trace(Visitor*) { }
+    virtual void trace(Visitor*) override { }
 };
 
 template<typename T>
 class Super : public GarbageCollected<Super<T> >, public Mixin {
     USING_GARBAGE_COLLECTED_MIXIN(Super);
 public:
-    virtual void trace(Visitor*);
+    virtual void trace(Visitor*) override;
     void clearWeakMembers(Visitor*);
 private:
     Member<HeapObject> m_obj;
@@ -30,14 +30,14 @@ private:
 template<typename T>
 class Sub : public Super<T> {
 public:
-    virtual void trace(Visitor* visitor);
+    virtual void trace(Visitor* visitor) override;
 private:
     Member<HeapObject> m_obj;
 };
 
 class HeapObject : public Sub<HeapObject> {
 public:
-    virtual void trace(Visitor*);
+    virtual void trace(Visitor*) override;
 private:
     Member<HeapObject> m_obj;
 };