Tie page visibility api to on demand pinger (#2818)
Tie page visibility api to on demand pinger (#2818)

Avoid making requests to the on demand reloader if the page is not visible.
BlancheXu/test,flybayer/next.js,callumlocke/next.js,flybayer/next.js,zeit/next.js,BlancheXu/test,azukaru/next.js,JeromeFitz/next.js,flybayer/next.js,kpdecker/next.js,azukaru/next.js,JeromeFitz/next.js,giacomorebonato/next.js,zeit/next.js,JeromeFitz/next.js,JeromeFitz/next.js,flybayer/next.js,nelak/next.js,azukaru/next.js,azukaru/next.js,zeit/next.js,BlancheXu/test