Oilpan: tidy up some inter-stack object references.

Source/core/css/parser/CSSParserObserverWrapper.h

Index: Source/core/css/parser/CSSParserObserverWrapper.h
diff --git a/Source/core/css/parser/CSSParserObserverWrapper.h b/Source/core/css/parser/CSSParserObserverWrapper.h
index 1aff9fcdb42e4478047de7b545a31f211f87cbcd..eb1ea2244e708eab318d91fca95d897c202483a3 100644
--- a/Source/core/css/parser/CSSParserObserverWrapper.h
+++ b/Source/core/css/parser/CSSParserObserverWrapper.h
@@ -10,9 +10,10 @@
 namespace blink {
 
 class CSSParserObserverWrapper {
+    STACK_ALLOCATED();
 public:
-    CSSParserObserverWrapper(CSSParserObserver& observer)
-    : m_observer(observer)
+    explicit CSSParserObserverWrapper(CSSParserObserver& observer)
+        : m_observer(observer)
     { }
 
     unsigned startOffset(const CSSParserTokenRange&);