Oilpan: have CSSImageGeneratorValue use SelfKeepAlive<>.

Source/core/css/CSSImageGeneratorValue.h

Index: Source/core/css/CSSImageGeneratorValue.h
diff --git a/Source/core/css/CSSImageGeneratorValue.h b/Source/core/css/CSSImageGeneratorValue.h
index 96907047d260c3fde3402618ac775f18d9b47d81..93f31566fa0a7f9ae85dab26b3bb628f2d7a439c 100644
--- a/Source/core/css/CSSImageGeneratorValue.h
+++ b/Source/core/css/CSSImageGeneratorValue.h
@@ -84,8 +84,7 @@ protected:
     // FIXME: Oilpan: when/if we can make the layoutObject point directly to the CSSImageGenerator value using
     // a member we don't need to have this hack where we keep a persistent to the instance as long as
     // there are clients in the LayoutObjectSizeCountMap.
-    GC_PLUGIN_IGNORE("366546")
-    OwnPtr<Persistent<CSSImageGeneratorValue>> m_keepAlive;
+    SelfKeepAlive<CSSImageGeneratorValue> m_keepAlive;
 #endif
 };