Remove an excessive mutable type specifier

[kuzkry]

The private mutable member of MockFunction cannot be reached from outside and all its
non-static member functions are not const. Alternatively, we could add const to every function of MockFunction, though it's a much worse idea I believe.

@adambadura - watch out, this will conflict with your #2350 if this gets merged first.

[gennadiycivil]

Thank your for this contribution. Could you please elaborate and provide more information on exactly the issue / problems this is trying to address.
What is the before/after effect this would create and how is this beneficial?
In other words, is there a concrete non-theoretical case where this can be illustrated?

[kuzkry]

Hmm, pardon me, actually nothing concrete comes to my mind. This is, I suppose, because of the pure nature of mutable which should be given only to make members of a const object modifiable. We shouldn't use it without a specific, exceptional need.
I've even digged into git history in order to find and ask an author but it turned out to be no-one specific, i.e. Abseil Team in de5be0. If this is possible for you to find them, please do.
Last but not least, I could apply the reversed logic and ask if there is a scenario that requires having this mutable ;) Perhaps, it's the right way to think about it.

[gennadiycivil]

Looking, stay tuned

[gennadiycivil]

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