This function was added during the Go 1.11 dev cycle and isn't part of
the API compatibility promise yet.

In the previous implementation, NewGCMWithNonceAndTagSize was being used
as a helper function for NewGCM, NewGCMWithTagSize, and NewGCMWithNonceSize.
With the removal of Nonce size from the name and parameters, we needed to
add an unexported helper function newGCMWithNonceAndTagSize.

Fixes #24977

Change-Id: Ie70f2a192d0556c4f890deb62e68cff6bbbccd33
Reviewed-on: https://go-review.googlesource.com/116435Reviewed-by: Filippo Valsorda <filippo@golang.org>
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>