Update audioStartTime and audioEndTime to be relative to start of audio stream

We want to switch to  stream-Relative (0-based) impl. since:
- In Web Audio and Media APIs (HTMLMediaElement.currentTime, AudioContext.currentTime, WebCodecs VideoFrame.timestamp), media timelines are always 0-based offsets relative to stream start, not the time origin.
- Immune to inter-process jitter since SODA and audio capture run in a separate utility/browser process. Translating stream offsets to the renderer's timeOrigin relies on estimating when IPC AudioStarted() arrived, which introduces IPC latency jitter. Stream-relative offsets are not prune to this and aligns with the raw audio frames.
- If SpeechRecognition is used with a pre-recorded MediaStreamTrack, a 0-based stream offset reflects the actual position in the audio track regardless of when the webpage was loaded.
- Being relative to performance.timeOrigin doesn't make sense in general for the Web Speech API because it assumes that the audio source is live. Since a SpeechRecognizer can also be created for a prerecorded media stream I think the timestamps on the speech recognition events should be relative to the position in that media stream.

See https://crbug.com/542330168 for more details.

Author: alan33d

explainers/speech-recognition-result-timestamps.md

@@ -17,10 +17,10 @@ We propose extending the `SpeechRecognitionResult` interface to include optional
 
 ```webidl
 partial interface SpeechRecognitionResult {
-    // Start timestamp of the audio segment in milliseconds (relative to time origin)
+    // Start timestamp of the audio segment in milliseconds (relative to the start of the audio stream)
     readonly attribute DOMHighResTimeStamp? audioStartTime;
 
-    // End timestamp of the audio segment in milliseconds (relative to time origin)
+    // End timestamp of the audio segment in milliseconds (relative to the start of the audio stream)
     readonly attribute DOMHighResTimeStamp? audioEndTime;
 };
 ```