This is I believe somewhat an extension of the Page Visibility API. Guarded by the ENABLE_HIDDEN_PAGE_DOM_TIMER_THROTTLING feature define.
Hm, I missed this one when I enabled page visibility. Will check this one.
After some checking - this functionality seems to be linked to setProcessSuppressionEnabled API in WebProcess which is enabled for MAC only. It is not clear for me if the process suppression is needed for GTK port, may be Carlos or Martin can comment on it.
I don't think this feature exists anymore in the code, see bug 126928.
I'm wrong about that, typo in my grepping! =(
I think it's safe to just turn this on.