[3P] Error: TypeError: undefined is not a function

## What's the issue?
We're getting a typeError inside of https://raw.githubusercontent.com/ampproject/amphtml/1907231757120/src/utils/rate-limit.js that's overwhelming our Sentry.io report.

undefined is not a function (evaluating 'a.setTimeout(e,c)')

Here's the code that's blowing up: ```
function fire(args) {
nextCallArgs = null;
// Lock the fire for minInterval milliseconds
locker = win.setTimeout(waiter, minInterval);
callback.apply(null, args);
}```

Here's the stack trace:

https://raw.githubusercontent.com/ampproject/amphtml/1907231757120/src/utils/rate-limit.js in fire at line 37:14
https://raw.githubusercontent.com/ampproject/amphtml/1907231757120/src/exponential-backoff.js at line 37:3
https://raw.githubusercontent.com/ampproject/amphtml/1907231757120/src/inabox/inabox-viewport.js in mo at line 228:7
https://raw.githubusercontent.com/ampproject/amphtml/1907231757120/src/inabox/inabox-viewport.js in getTargetRect at line 209:9
https://raw.githubusercontent.com/ampproject/amphtml/1907231757120/ads/inabox/position-observer.js at line 100:21
https://raw.githubusercontent.com/ampproject/amphtml/1907231757120/src/observable.js in fire at line 81:7
https://raw.githubusercontent.com/ampproject/amphtml/1907231757120/ads/inabox/position-observer.js at line 69:9
https://raw.githubusercontent.com/ampproject/amphtml/1907231757120/src/utils/rate-limit.js in fire at line 39:5
https://raw.githubusercontent.com/ampproject/amphtml/1907231757120/ads/inabox/position-observer.js in d at line 44:20
/vendors~app.dc245b37b76d93276af6.js in r at line 231:11581

## How do we reproduce the issue?
I'm not exactly sure...we're just getting hammered with the errors on our error reporting service.

## What browsers are affected?

It appears to be only Safari 6.2.8, and Mac OS X 10.8.5

## Which AMP version is affected?
I'm not sure because I don't know how to reproduce.

Start with src/utils/rate-limit.js, especially fire, and trace the call path through src/exponential-backoff.js, src/inabox/inabox-viewport.js, and ads/inabox/position-observer.js. Try to reproduce the timer failure in Safari 6.2.8 on Mac OS X 10.8.5; done means identifying the cause and verifying the behavior no longer produces the reported TypeError.

Written by the indexing model from the issue text.