Oilpan: tidy up some inter-stack object references.

Source/modules/crypto/CryptoKey.cpp

Index: Source/modules/crypto/CryptoKey.cpp
diff --git a/Source/modules/crypto/CryptoKey.cpp b/Source/modules/crypto/CryptoKey.cpp
index 98b5597680063c38ced2547d81f75a8c1232877e..94d03ed7a665078edf111bf6ff62c074dd33ce39 100644
--- a/Source/modules/crypto/CryptoKey.cpp
+++ b/Source/modules/crypto/CryptoKey.cpp
@@ -130,6 +130,9 @@ public:
     }
 
 private:
+    // Oilpan: CSSParserObserver is stack allocated and stack scanning will
+    // separately locate it.
+    GC_PLUGIN_IGNORE("509911")
     V8ObjectBuilder& m_builder;
 };