We recently adopted RunningBoard on macOS and our child processes priorities are now tied to RunningBoard assertions that we hold on their behalf. This means that WebProcesses backing background tabs now get a much lower process priority than they used to. Given that the responsiveness timeout is fairly short (3 seconds), this can lead to false positives.

To address the issue, we now hold a foreground assertion during the responsiveness check instead of the implicit background one taken by sendWithAsyncReply().