GC plugin: remove mixin trace override restriction.

tools/clang/blink_gc_plugin/tests/class_multiple_trace_bases.cpp

 // Copyright 2014 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.
 
 #include "class_multiple_trace_bases.h"
 
 namespace blink {
 
 void Base::trace(Visitor* visitor) { }
 
-void Mixin::trace(Visitor* visitor) { }
+void Mixin1::trace(Visitor* visitor) { }
 
-// Missing: Derived::trace(visitor)
+void Mixin2::trace(Visitor* visitor) { }
+
+// Missing: void Derived1::trace(Visitor* visitor);
+
+void Derived2::trace(Visitor* visitor) {
+    Base::trace(visitor);
+    Mixin1::trace(visitor);
+}
 
 }