Browser API: implement iframe.reload()
fixes#8575
The implementation is naive, and doesn't support the `hardreload` parameter.
And for the test, I'm not sure how else I can test the reload.
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fix intermittent Option::unwrap in timers
fixes intermittent #8616
This intermittent indicates real problem in code.
Lets consider such code:
```javascript
// timer 1
setTimeout(function() {
//timer 2
setTimeout(function() {}, 0);
}, 0);
```
When we receive event to fire timer 1 it will be selected and extracted from active timers list in fire_timer function. During timer 1 handler execution we will schedule timer 2 and request timer event for it. But it will be executed during same fire_timer call because of 0 timeout. And as a result we will have empty timers list and expecting event for timer 2 that will crash in assert.
I'm not sure that all I've written is clear, but we have something like this:
```
install timer 1 -> [1] in timers list
push and expect timer event 1 -> expected_event=1
received timer event 1
fire_timer()
select timer 1 to execute -> [] in timers list
execute timer 1 handler
install timer 2 -> [2] in timers list
push and expect timer event 2 -> expected_event=2
select timer 2 to execute (because of 0 timeout) -> [] in tiemrs list
execute timer 2 handler
expected_event=2 is dangling
received timer event 2
fire_timer() -> BOOM
```
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Implement Range#deleteContents
Sadly calling ExtractContents and discarding the result doesn't do the right thing.
It may be worth having a CutContents method that takes an `Option<DocumentFragment>` and switch the behavior based on it, to share the code between DeleteContents and ExtractContents, like what Gecko does. Maybe a followup.
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