const-qualify some data members
const-qualify some data members

Summary: The `size_` and `upperBound_` fields can be const-qualified.

Reviewed By: ot

Differential Revision: D8734943

fbshipit-source-id: ced8a7c96442772a37bb8be373c71b557598257f

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly