[dev.boringcrypto] crypto/internal/boring: fix aesCipher implementation of gcmAble

[dev.boringcrypto] crypto/internal/boring: fix aesCipher implementation of gcmAble

In CL 48510 the gcmAble interface was changed to include the tag size.
The BoringCrypto aesCipher implementation wasn't updated, causing a
failed type assertion and consequently a performance degradation.

Change-Id: Ie5cff9ef242218d60f82795f3eb6760a57fe06f5
Reviewed-on: https://go-review.googlesource.com/127821
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Adam Langley <agl@golang.org>