When we set the MockMSE to run in the GPU Process we get failures:
media/media-source/media-source-first-append-not-starting-at-zero.html 

```
Enqueued so far: (expecting no frames yet)
+ 10{PTS({5/1 = 5.000000}), DTS({5/1 = 5.000000}), duration({1/1 = 1.000000}), flags(1), generation(0)}
+ 11{PTS({6/1 = 6.000000}), DTS({6/1 = 6.000000}), duration({1/1 = 1.000000}), flags(0), generation(0)}
+ 12{PTS({7/1 = 7.000000}), DTS({7/1 = 7.000000}), duration({1/1 = 1.000000}), flags(0), generation(0)}
+ 13{PTS({8/1 = 8.000000}), DTS({8/1 = 8.000000}), duration({1/1 = 1.000000}), flags(0), generation(0)}
+ 14{PTS({9/1 = 9.000000}), DTS({9/1 = 9.000000}), duration({1/1 = 1.000000}), flags(0), generation(0)}
RUN(sourceBuffer.appendBuffer(syncSampleRun(0, 5)))
EVENT(updateend)
Enqueued now:
```