GC plugin: remove mixin trace override restriction.

tools/clang/blink_gc_plugin/tests/class_multiple_trace_bases.h

Index: tools/clang/blink_gc_plugin/tests/class_multiple_trace_bases.h
diff --git a/tools/clang/blink_gc_plugin/tests/class_multiple_trace_bases.h b/tools/clang/blink_gc_plugin/tests/class_multiple_trace_bases.h
index b362ec921b5c9ff97f05666124afad954b0db5e0..133f0061b5d9f7a5b41c77f0138462a567d56dbb 100644
--- a/tools/clang/blink_gc_plugin/tests/class_multiple_trace_bases.h
+++ b/tools/clang/blink_gc_plugin/tests/class_multiple_trace_bases.h
@@ -14,16 +14,26 @@ public:
     virtual void trace(Visitor*);
 };
 
-class Mixin : public GarbageCollectedMixin {
+class Mixin1 : public GarbageCollectedMixin {
 public:
     void trace(Visitor*);
 };
 
-class Derived : public Base, public Mixin {
-    USING_GARBAGE_COLLECTED_MIXIN(HeapObject);
+class Mixin2 : public GarbageCollectedMixin {
+public:
+    void trace(Visitor*);
+};
+
+class Derived1 : public Base, public Mixin1 {
+    USING_GARBAGE_COLLECTED_MIXIN(Derived1);
     // Requires trace method.
 };
 
+class Derived2 : public Base, public Mixin1, public Mixin2 {
+    USING_GARBAGE_COLLECTED_MIXIN(Derived2);
+    void trace(Visitor*) override;
+};
+
 }
 
 #endif