Layout Test media/media-continues-playing-after-replace-source.html is flaky

[Matt Lewis]

The following layout test is a flaky timeout on macOS Debug WK1

media/media-continues-playing-after-replace-source.html

The first timeout occurred with this build:
https://build.webkit.org/builders/Apple%20High%20Sierra%20Debug%20WK1%20(Tests)/builds/2282
https://build.webkit.org/results/Apple%20High%20Sierra%20Debug%20WK1%20(Tests)/r228235%20(2282)/results.html

Flakiness Dashboard:

https://webkit-test-results.webkit.org/dashboards/flakiness_dashboard.html#showAllRuns=true&tests=media%2Fmedia-continues-playing-after-replace-source.html

Current Build:
https://build.webkit.org/results/Apple%20High%20Sierra%20Debug%20WK1%20(Tests)/r228417%20(2364)/results.html
https://build.webkit.org/builders/Apple%20High%20Sierra%20Debug%20WK1%20(Tests)/builds/2364

[Alexey Proskuryakov]

This appears to be pretty rare, so hard to tell if this is a true regression or not.