Blink GC plugin: require a trace method if a class derives multiple bases that need tracing.

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 "non_virtual_trace.h"
+#include "class_multiple_trace_bases.h"
 
 namespace WebCore {
 
-void A::trace(Visitor* visitor)
-{
-}
+void Base::trace(Visitor* visitor) { }
 
-void C::trace(Visitor* visitor)
-{
-    B::trace(visitor);
-}
+void Mixin::trace(Visitor* visitor) { }
+
+// Missing: Derived::trace(visitor)
 
 }