BlinkGCPlugin: Support overloaded trace() call from template.

tools/clang/blink_gc_plugin/tests/traceimpl_overloaded.cpp

+// Copyright 2015 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 "traceimpl_overloaded.h"
+
+namespace blink {
+
+void ExternBase::trace(Visitor* visitor) {
+  traceImpl(visitor);
+}
+
+void ExternBase::trace(InlinedGlobalMarkingVisitor visitor) {
+  traceImpl(visitor);
+}
+
+template <typename VisitorDispatcher>
+inline void ExternBase::traceImpl(VisitorDispatcher visitor) {
+  visitor->trace(x_base_);
+}
+
+void ExternDerived::trace(Visitor* visitor) {
+  traceImpl(visitor);
+}
+
+void ExternDerived::trace(InlinedGlobalMarkingVisitor visitor) {
+  traceImpl(visitor);
+}
+
+template <typename VisitorDispatcher>
+inline void ExternDerived::traceImpl(VisitorDispatcher visitor) {
+  visitor->trace(x_derived_);
+  ExternBase::trace(visitor);
+}
+
+}