This is a further enhancement to #3690
It reduces binary size of amp-analytics by 9K under most of browsers.
As a trade-off, it introduces a HTTP round trip for some old browsers, such as IE11 and Android 4.x system browser.

Plan:

• Have a local implementation of base64()
• Strip out base64 code from closure sha384 lib
• Serve the closure lib in CDN (@cramforce any example how do we host such a file?)
• Lazy load it in CryptoService code

@jridgewell