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

Source/core/editing/InputMethodController.h

Index: Source/core/editing/InputMethodController.h
diff --git a/Source/core/editing/InputMethodController.h b/Source/core/editing/InputMethodController.h
index 9bb3c29bdd642a158b600aabb355200bc750beed..53a486a8f10517e6232ea4ff824bbcbe6984f237 100644
--- a/Source/core/editing/InputMethodController.h
+++ b/Source/core/editing/InputMethodController.h
@@ -86,11 +86,12 @@ public:
 private:
     class SelectionOffsetsScope {
         WTF_MAKE_NONCOPYABLE(SelectionOffsetsScope);
+        STACK_ALLOCATED();
     public:
-        SelectionOffsetsScope(InputMethodController*);
+        explicit SelectionOffsetsScope(InputMethodController*);
         ~SelectionOffsetsScope();
     private:
-        InputMethodController* m_inputMethodController;
+        RawPtrWillBeMember<InputMethodController> m_inputMethodController;
         const PlainTextRange m_offsets;
     };
     friend class SelectionOffsetsScope;