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

tools/clang/blink_gc_plugin/tests/class_requires_trace_method.h

Index: tools/clang/blink_gc_plugin/tests/class_requires_trace_method.h
diff --git a/tools/clang/blink_gc_plugin/tests/class_requires_trace_method.h b/tools/clang/blink_gc_plugin/tests/class_requires_trace_method.h
index 82cad15e884d799d066537ecb293ab5b61eb2a65..782d80e141dafc43feff82718ed442b9a97ada26 100644
--- a/tools/clang/blink_gc_plugin/tests/class_requires_trace_method.h
+++ b/tools/clang/blink_gc_plugin/tests/class_requires_trace_method.h
@@ -22,6 +22,38 @@ private:
     PartObject m_part;
 };
 
+class Mixin : public GarbageCollectedMixin {
+public:
+  virtual void trace(Visitor*) override;
+  Member<Mixin> m_self;
+};
+
+class HeapObjectMixin : public GarbageCollected<HeapObjectMixin>, public Mixin {
+  USING_GARBAGE_COLLECTED_MIXIN(HeapObjectMixin);
+};
+
+class Mixin2 : public Mixin {
+public:
+  virtual void trace(Visitor*) override;
+};
+
+class HeapObjectMixin2
+    : public GarbageCollected<HeapObjectMixin2>, public Mixin2 {
+  USING_GARBAGE_COLLECTED_MIXIN(HeapObjectMixin2);
+};
+
+class Mixin3 : public Mixin {
+public:
+  virtual void trace(Visitor*) override;
+};
+
+class HeapObjectMixin3
+    : public GarbageCollected<HeapObjectMixin3>, public Mixin {
+  USING_GARBAGE_COLLECTED_MIXIN(HeapObjectMixin2);
+public:
+  virtual void trace(Visitor*) override;
+};
+
 }
 
 #endif