Oilpan: clarify why FrameOwner is a GC mixin.

Source/core/frame/FrameOwner.h

 // 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.
 
 #ifndef FrameOwner_h
 #define FrameOwner_h
 
 #include "core/dom/SandboxFlags.h"
 #include "platform/heap/Handle.h"
 
 namespace blink {
 
+// Oilpan: all FrameOwner instances are GCed objects. FrameOwner additionally
+// derives from GarbageCollectedMixin so that Member<FrameOwner> references can
+// be kept (e.g., Frame::m_owner.)
 class FrameOwner : public WillBeGarbageCollectedMixin {
 public:
     virtual ~FrameOwner() { }
     virtual void trace(Visitor*) { }
 
     virtual bool isLocal() const = 0;
 
     virtual SandboxFlags sandboxFlags() const = 0;
     virtual void dispatchLoad() = 0;
 };
 
 } // namespace blink
 
 #endif // FrameOwner_h