Use crypto.subtle instead of closure's sha384 where available

[cramforce]

And late load closure implementation when needed (none of the current major browsers).

The resulting strings must by byte-for-byte compatible.

CC @lannka

[ericlindley-g]

@cramforce — slotting in backlog, but please correct/update as needed

[lannka]

Did some experiments.

• Latest Chrome, FireFox, Edge, Opera have full support of window.crypto.subtle.digest();
• Safari supports it with window.crypto.webkitSubtle.digest.
• IE11 supports it with window.msCrypto.subtle.digest(); however, SHA-1 is not supported; and the return is not a Promise

Basically all browsers AMP supports have usable implementations except Android 4.x system browsers.

@cramforce Do you think it's still necessary to have the fallback of late loading closure implementation?

[cramforce]

What I think we should do is:

• create a CryptoService or similar
• use that across this, analytics, CID, etc.
• have it use builtin by default
• and late load closure otherwise.

[lannka]

The work is done except the lazy loading part. Can open a separate issue if wanted.

[jridgewell]

Can open a separate issue if wanted.

Please.