Bug 252886

Per spec MSE spec and the Coded Frame Processing algorithm [1]
"5. if the media segment contains data beyond the current duration, then run the duration change algorithm with new duration set to the maximum of the current duration and the [[group end timestamp]]."

Which occurs once all frames of a buffer added to a source buffer have been processed.

However, the SourceBufferPrivate.cpp didReceiveSample performs this step for every single frame received.
https://jyavenard.github.io/htmltests/tests/mse_webm/vorbis_timestampOffset.html
(need bug 252782 fixed)

results in the following result:
```
1237: ok(true) Receive a sourceopen event
1303: got abort event
1303: got emptied event
1306: got loadstart event
1371: got durationchange event
1373: got loadedmetadata event
1374: got durationchange event
1381: Append a media segment.
1390: SourceBuffer buffered ranges grew from TimeRanges:  to TimeRanges: [0, 1.3392199546485262)
1390: Append a media segment.
1439: got durationchange event
1440: got loadeddata event
1440: got canplay event
1441: got canplaythrough event
1457: SourceBuffer buffered ranges grew from TimeRanges: [0, 1.3392199546485262) to TimeRanges: [0, 2.678440657773526)
1458: Append a media segment.
1520: got durationchange event
1521: got durationchange event
1522: got durationchange event
1522: got durationchange event
1523: got durationchange event
1523: got durationchange event
1524: got durationchange event
1525: got durationchange event
1526: got durationchange event
1526: got durationchange event
1527: got durationchange event
1528: got durationchange event
1529: got durationchange event
1530: got durationchange event
1530: got durationchange event
1531: got durationchange event
1532: got durationchange event
1532: got durationchange event
1533: got durationchange event
1534: got durationchange event
1535: got durationchange event
1537: got durationchange event
1538: got durationchange event
1539: got durationchange event
1539: got durationchange event
1540: got durationchange event
1541: got durationchange event
```

We get one `durationchange` event for every single frame added to the second buffer after the initial `durationchange` fired following the first segment.

Only one `durationchange` event should be fired.

[1] https://w3c.github.io/media-source/#sourcebuffer-coded-frame-processing the last step is:
[2] https://searchfox.org/wubkat/rev/e4f3157afc6ca0cf36b48e57e2337b4ceb66d459/Source/WebCore/platform/graphics/SourceBufferPrivate.cpp#1088-1093