Oilpan: add missing leftmost trace()s for GC mixins.

Source/web/ContextFeaturesClientImpl.cpp

 /*
  * Copyright (C) 2012 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 71 matching lines @@
 
     Entry& entryFor(ContextFeatures::FeatureType type)
     {
         size_t index = static_cast<size_t>(type);
         ASSERT_WITH_SECURITY_IMPLICATION(index < ContextFeatures::FeatureTypeSize);
         return m_entries[index];
     }
 
     void validateAgainst(Document*);
 
+    virtual void trace(Visitor* visitor) override
+    {
+        DocumentSupplement::trace(visitor);
+    }
+
 private:
     String m_domain;
     Entry m_entries[ContextFeatures::FeatureTypeSize];
 };
 
 const char* ContextFeaturesCache::supplementName()
 {
     return "ContextFeaturesCache";
 }
 
@@ skipping 43 matching lines @@
     case ContextFeatures::MutationEvents:
         return frame->permissionClient()->allowMutationEvents(defaultValue);
     case ContextFeatures::PushState:
         return frame->permissionClient()->allowPushState();
     default:
         return defaultValue;
     }
 }
 
 } // namespace blink