[whatwg] Adding a btoa overload that takes Uint8Array

[Boris Zbarsky]

The problem I'm trying to solve is sending Unicode text to consumers who 
need base64-encoded input.  Right now the only "sane" way to do it (and 
I quote "sane" for obvious reasons) is something like the example at 
https://developer.mozilla.org/en-US/docs/DOM/window.btoa#Unicode_Strings

It seems like it would be better if the output of a TextEncoder could be 
passed directly to btoa.  But for that we need an overload of btoa that 
takes a Uint8Array.

Thoughts?

-Boris