Add one more webkit-specific whitelist in web-sys

[alexcrichton]

Adds OfflineAudioContext to the list of types that may have the webkit prefix, as mentioned here

[alexcrichton]

I'm gonna hold off on merging this until @tat-m3dicine confirms this works

[tat-m3dicine]

Beautiful work! I can instantiate webkitAudioOfflineContext on Safari without any problem!
Just making sure that I followed the step on contributing.md correctly, I've done following steps:

1. cd crates/web-sys && cargo build
2. import and use web_sys::OfflineAudioContext in examples/webaudio/lib.rs

[alexcrichton]

Nice, thanks for confirming!