GC plugin: enable plugin on anonymous namespaces.

tools/clang/blink_gc_plugin/tests/stack_allocated.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 "stack_allocated.h"
 
 namespace blink {
 
+// Verify that anon namespaces are checked.
+namespace {
+
+class AnonStackObject : public StackObject {
+public:
+    HeapObject* m_obj;
+};
+
+}
+
 void HeapObject::trace(Visitor* visitor)
 {
 }
 
 }