Add currentFrame to AudioWorkletGlobalScope

[rtoy]

The AudioWorkletGlobalScope has currentTime to let the processor know the current time. It would also be very nice to have currentFrame that is the frame counter where currentTime = currentFrame/sampleRate.

For algorithms that need precise timing and complex computations wrt to time, using frames reduces round-off errors.

In Chrome, currentTime used to be accumulated each render quantum, causing a small round-off each time. This slight drift was noticeable after even a short time. Switching Chrome's internal implementation to count frames got rid a huge class of round-off errors.

Doing the same in Chrome's AudioParam implementation greatly reduced the small, but subtle issues in timing. Before fixing this, @hoch had an example where his synthesizer would suddenly stop producing output because of round-off in the audio param implementation. Changing the implementation to work with frames instead of time fixed this and a bunch of other issues.

[padenot]

Agreed. Gecko does everything in frames internally, exactly for this reason.