This is more representative of what actually occurs, as web does support remote URLs which wouldn't need a romfs callback. This paves for easy future support of this with a call like 'OpenPageRemote' or similar.

This is a function that definitely doesn't always have a non-modifying
behavior across all implementations, so this should be made non-const.

This gets rid of the need to mark data members as mutable to work around
the fact mutating data members needs to occur.