Oilpan: tidy up definition of some stack-allocated scope objects.

Source/core/editing/Editor.h

Index: Source/core/editing/Editor.h
diff --git a/Source/core/editing/Editor.h b/Source/core/editing/Editor.h
index c08b4198a8b052fa09b5c2fdf1bde8f11bc38a6a..6335a900b976bfbbd53d284f0faa40f0efa53000 100644
--- a/Source/core/editing/Editor.h
+++ b/Source/core/editing/Editor.h
@@ -226,11 +226,12 @@ public:
 
     class RevealSelectionScope {
         WTF_MAKE_NONCOPYABLE(RevealSelectionScope);
+        STACK_ALLOCATED();
     public:
-        RevealSelectionScope(Editor*);
+        explicit RevealSelectionScope(Editor*);
         ~RevealSelectionScope();
     private:
-        Editor* m_editor;
+        RawPtrWillBeMember<Editor> m_editor;
     };
     friend class RevealSelectionScope;