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.h

Index: tools/clang/blink_gc_plugin/tests/class_multiple_trace_bases.h
diff --git a/tools/clang/blink_gc_plugin/tests/garbage_collected_mixin.h b/tools/clang/blink_gc_plugin/tests/class_multiple_trace_bases.h
similarity index 60%
copy from tools/clang/blink_gc_plugin/tests/garbage_collected_mixin.h
copy to tools/clang/blink_gc_plugin/tests/class_multiple_trace_bases.h
index 127199de44ff70e4586e9b1acf1425e583a9b290..afbf39ea02c5e67f34f65330cc1e309e5eb0287a 100644
--- a/tools/clang/blink_gc_plugin/tests/garbage_collected_mixin.h
+++ b/tools/clang/blink_gc_plugin/tests/class_multiple_trace_bases.h
@@ -2,26 +2,26 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-#ifndef GARBAGE_COLLECTED_MIXIN_H_
-#define GARBAGE_COLLECTED_MIXIN_H_
+#ifndef CLASS_MULTIPLE_TRACE_BASES_H_
+#define CLASS_MULTIPLE_TRACE_BASES_H_
 
 #include "heap/stubs.h"
 
 namespace WebCore {
 
+class Base : public GarbageCollected<Base> {
+public:
+    virtual void trace(Visitor*);
+};
+
 class Mixin : public GarbageCollectedMixin {
 public:
     void trace(Visitor*);
-private:
-    Member<Mixin> m_self;
 };
 
-class HeapObject : public GarbageCollected<HeapObject>, public Mixin {
+class Derived : public Base, public Mixin {
     USING_GARBAGE_COLLECTED_MIXIN(HeapObject);
-public:
-    void trace(Visitor*);
-private:
-    Member<Mixin> m_mix;
+    // Requires trace method.
 };
 
 }