GC plugin: remove mixin trace override restriction.

tools/clang/blink_gc_plugin/tests/class_multiple_trace_bases.cpp

Index: tools/clang/blink_gc_plugin/tests/class_multiple_trace_bases.cpp
diff --git a/tools/clang/blink_gc_plugin/tests/class_multiple_trace_bases.cpp b/tools/clang/blink_gc_plugin/tests/class_multiple_trace_bases.cpp
index 4afc011c6bb47f231c160e7cc0a8c2d5fd475463..5bb87c92d60fd5271556c62a504de680eaf13c7d 100644
--- a/tools/clang/blink_gc_plugin/tests/class_multiple_trace_bases.cpp
+++ b/tools/clang/blink_gc_plugin/tests/class_multiple_trace_bases.cpp
@@ -8,8 +8,15 @@ 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);
+}
 
 }