Add aes128gcm

* Update the HTTP Encrypted Content Encoding implementation to support
aes128gcm, as defined in the 9th version of the draft [1].
* Update the Web Push implementation to use aes128gcm, as defined in the
9th version of the draft [2].

[1] https://tools.ietf.org/html/draft-ietf-httpbis-encryption-encoding-09
[2] https://tools.ietf.org/html/draft-ietf-webpush-encryption-09