[WeakMemoryCache] Field-Trial: Make Reference from MemoryCache to Resource weak

third_party/WebKit/Source/core/fetch/MemoryCache.h

 /*
     Copyright (C) 1998 Lars Knoll (knoll@mpi-hd.mpg.de)
     Copyright (C) 2001 Dirk Mueller <mueller@kde.org>
     Copyright (C) 2004, 2005, 2006, 2007, 2008 Apple Inc. All rights reserved.
 
     This library is free software; you can redistribute it and/or
     modify it under the terms of the GNU Library General Public
     License as published by the Free Software Foundation; either
     version 2 of the License, or (at your option) any later version.
 
@@ skipping 68 matching lines @@
     bool m_inLiveDecodedResourcesList;
     unsigned m_accessCount;
     double m_lastDecodedAccessTime; // Used as a thrash guard
 
     Member<MemoryCacheEntry> m_previousInLiveResourcesList;
     Member<MemoryCacheEntry> m_nextInLiveResourcesList;
     Member<MemoryCacheEntry> m_previousInAllResourcesList;
     Member<MemoryCacheEntry> m_nextInAllResourcesList;
 
 private:
-    explicit MemoryCacheEntry(Resource* resource)
-        : m_inLiveDecodedResourcesList(false)
-        , m_accessCount(0)
-        , m_lastDecodedAccessTime(0.0)
-        , m_previousInLiveResourcesList(nullptr)
-        , m_nextInLiveResourcesList(nullptr)
-        , m_previousInAllResourcesList(nullptr)
-        , m_nextInAllResourcesList(nullptr)
-        , m_resource(resource)
-    {
-    }
+    explicit MemoryCacheEntry(Resource*);
 
+    // Only one of |m_resource| or |m_resourceWeak| is used, based on
+    // RuntimeEnabledFeatures::weakMemoryCacheEnabled().
+    // TODO(hiroshige): this switching mechanism is added for a field trial
+    // for Weak MemoryCache (https://crbug.com/603462) and should be reverted
+    // at least before going stable.
     Member<Resource> m_resource;
+    WeakMember<Resource> m_resourceWeak;
 };
 
 WILL_NOT_BE_EAGERLY_TRACED_CLASS(MemoryCacheEntry);
 
 // MemoryCacheLRUList is used only in MemoryCache class, but we don't make
 // MemoryCacheLRUList an inner struct of MemoryCache because we can't define
 // VectorTraits for inner structs.
 struct MemoryCacheLRUList final {
     DISALLOW_NEW_EXCEPT_PLACEMENT_NEW();
 public:
@@ skipping 197 matching lines @@
 // Returns the global cache.
 CORE_EXPORT MemoryCache* memoryCache();
 
 // Sets the global cache, used to swap in a test instance. Returns the old
 // MemoryCache object.
 CORE_EXPORT MemoryCache* replaceMemoryCacheForTesting(MemoryCache*);
 
 } // namespace blink
 
 #endif