Fix typings for several ElementArrayFinder methods

[inad9300]

Some of the callback functions had an incorrect ? for their arguments.

Done for #4634.

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[inad9300]

I've signed the agreement thing... Am I now an official non-paid Google employee?

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CLAs look good, thanks!

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CLAs look good, thanks!

[tymfear]

Why does the index parameter should be required?

[inad9300]

Removing the question mark does not mean that you are forced to accept index as a parameter in your callback. This is the whole point of #4634.

[tymfear]

Gotcha! Thanks a lot!