BlinkGCPlugin: Handle omitted trace() in intermediary classes.

tools/clang/blink_gc_plugin/tests/traceimpl_omitted_trace.h

 // 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.
 
-#ifndef TRACEIMPL_DEPENDENT_SCOPE_H_
-#define TRACEIMPL_DEPENDENT_SCOPE_H_
+#ifndef TRACEIMPL_OMITTED_TRACE_H_
+#define TRACEIMPL_OMITTED_TRACE_H_
 
 #include "heap/stubs.h"
 
 namespace blink {
 
-class X : public GarbageCollected<X> {
- public:
-  virtual void trace(Visitor*) {}
-};
-
-template <typename T>
-class Base : public GarbageCollected<Base<T> > {
+class A : public GarbageCollected<A> {
  public:
   virtual void trace(Visitor* visitor) { traceImpl(visitor); }
   virtual void trace(InlinedGlobalMarkingVisitor visitor) {
     traceImpl(visitor);
   }
 
  private:
   template <typename VisitorDispatcher>
   void traceImpl(VisitorDispatcher visitor) {}
 };
 
-template <typename T>
-class Derived : public Base<T> {
+class B : public A {
+  // trace() isn't necessary because we've got nothing to trace here.
+};
+
+class C : public B {
  public:
   void trace(Visitor* visitor) override { traceImpl(visitor); }
   void trace(InlinedGlobalMarkingVisitor visitor) override {
-    traceImpl(visitor);
-  }
-
- private:
-  template <typename VisitorDispatcher>
-  void traceImpl(VisitorDispatcher visitor) {
-    Base<T>::trace(visitor);
-  }
-};
-
-template <typename T>
-class DerivedMissingTrace : public Base<T> {
- public:
-  void trace(Visitor* visitor) override { traceImpl(visitor); }
-  void trace(InlinedGlobalMarkingVisitor visitor) override {
     traceImpl(visitor);
   }
 
  private:
   template <typename VisitorDispatcher>
   void traceImpl(VisitorDispatcher visitor) {
-    // Missing Base<T>::trace(visitor).
+    // B::trace() is actually A::trace(), and in certain cases we only get
+    // limited information like "there is a function call that will be resolved
+    // to A::trace()". We still want to mark B as traced.
+    B::trace(visitor);
   }
 };
 
 }
 
-#endif  // TRACEIMPL_DEPENDENT_SCOPE_H_
+#endif  // TRACEIMPL_OMITTED_TRACE_H_