Oilpan: have CSSImageGeneratorValue use SelfKeepAlive<>.

Source/core/css/CSSImageGeneratorValue.cpp

Index: Source/core/css/CSSImageGeneratorValue.cpp
diff --git a/Source/core/css/CSSImageGeneratorValue.cpp b/Source/core/css/CSSImageGeneratorValue.cpp
index 67a8ef400fbf233467e0b59debaadffe2fb00bfa..c8deda1faed2cadad8adcc538d8827c612dcb18c 100644
--- a/Source/core/css/CSSImageGeneratorValue.cpp
+++ b/Source/core/css/CSSImageGeneratorValue.cpp
@@ -50,7 +50,7 @@ void CSSImageGeneratorValue::addClient(LayoutObject* layoutObject, const IntSize
 #else
     if (m_clients.isEmpty()) {
         ASSERT(!m_keepAlive);
-        m_keepAlive = adoptPtr(new Persistent<CSSImageGeneratorValue>(this));
+        m_keepAlive = this;
     }
 #endif
 
@@ -89,7 +89,7 @@ void CSSImageGeneratorValue::removeClient(LayoutObject* layoutObject)
 #else
     if (m_clients.isEmpty()) {
         ASSERT(m_keepAlive);
-        m_keepAlive = nullptr;
+        m_keepAlive.clear();
     }
 #endif
 }