Oilpan: Fix a cycle caused by SVGPropertyTearOffBase::m_contextElement

Source/core/svg/properties/SVGAnimatedProperty.h

 /*
  * Copyright (C) 2013 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.
 G*     * Redistributions in binary form must reproduce the above
@@ skipping 83 matching lines @@
     {
     }
 
 protected:
     SVGAnimatedPropertyBase(AnimatedPropertyType, SVGElement*, const QualifiedName& attributeName);
 
 private:
     const AnimatedPropertyType m_type;
     bool m_isReadOnly;
 
-    // This reference is kept alive from V8 wrapper
-    // TODO(oilpan): This should be a Member. Currently we cannot do it because
-    // it creates a cycle as follows:
-    // SVGInterporation =(Persistent)=>
-    // SVGAnimatedProperty =(Member)=>
-    // SVGElement =(Member)=>
-    // ElementRareData =(Member)=>
-    // ElementAnimations =(part of object)=>
-    // CSSAnimations =(part of object)=>
-    // CSSAnimationUpdate =(Member)=>
-    // NewTransition =(Member)=>
-    // InertEffect =(Member)=>
-    // EffectModel =(RefPtr)=>
-    // InterpolationEffect =(RefPtr)=>
-    // InterpolationRecord =(RefPtr)=>
-    // SVGInterpolation
+    // This raw pointer is safe since the SVG element is guaranteed to be kept
+    // alive by a V8 wrapper.
+    GC_PLUGIN_IGNORE("crbug.com/528275")
     SVGElement* m_contextElement;
 
     const QualifiedName& m_attributeName;
 };
 
 template <typename Property>
 class SVGAnimatedPropertyCommon : public SVGAnimatedPropertyBase {
 public:
     Property* baseValue()
     {
@@ skipping 205 matching lines @@
     // When animated:
     //     m_animValTearOff targets m_currentValue.
     //     m_baseValTearOff targets m_baseValue.
     RefPtrWillBeMember<TearOffType> m_baseValTearOff;
     RefPtrWillBeMember<TearOffType> m_animValTearOff;
 };
 
 } // namespace blink
 
 #endif // SVGAnimatedProperty_h