Check for AudioContext to enable generating audio tracks with it

[youennf]

WebKit exposes webkitAudioContext which is slightly different from AudioContext.
For WebRTC tests, it is simpler to rely on getUserMedia tests with mock active.

[alvestrand]

It's good to make explicit that this way is only supposed to be used on standards-compliant WebAudio.

[alvestrand]

@nils-ohlmeier can you figure out what failed on Firefox? (if it still fails)

[wpt-pr-bot]

There are no reviewers for this pull request besides its author. Please reach out on W3C's irc server (irc.w3.org, port 6665) on channel #testing (web client) to get help with this. Thank you!

[nils-ohlmeier]

Locally the tests them self appear to work fine with and without this change.

But the logs collected here are way to verbose. And they reach the defined max output, so we can't see what actually happens at the end. But I'm failing to locate where the logs strings are set for WTP. Which WPT expert might be able to help locating that?

[nils-ohlmeier]

@foolip @jgraham does either of you know where the Firefox logging variables get set? I think we should remove the ICE logging, as they are not very useful on WPT. But the logs are way to big to be useful.

[foolip]

I defer to @jgraham on that.

[youennf]

OK, let's go with this PR then. I do not think this change should change much in Firefox.

[youennf]

FWIW, I think there is some flakiness in Firefox bot since Firefox is defining AudioContext and so this change should be a no op.