Auto merge of #8768 - vegayours:8616_intermittent_option_unwrap_in_timers, r=jdm

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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components/script/timers.rs

@@ -23,7 +23,7 @@ use std::rc::Rc;
 use util::mem::HeapSizeOf;
 use util::str::DOMString;
 
-#[derive(JSTraceable, PartialEq, Eq, Copy, Clone, HeapSizeOf, Hash, PartialOrd, Ord)]
+#[derive(JSTraceable, PartialEq, Eq, Copy, Clone, HeapSizeOf, Hash, PartialOrd, Ord, Debug)]
 pub struct TimerHandle(i32);
 
 #[derive(JSTraceable, HeapSizeOf)]
@@ -253,16 +253,22 @@ impl ActiveTimers {
         // Since the event id was the expected one, at least one timer should be due.
         assert!(base_time >= self.timers.borrow().last().unwrap().next_call);
 
+        // select timers to run to prevent firing timers
+        // that were installed during fire of another timer
+        let mut timers_to_run: Vec<Timer> = Vec::new();
+
         loop {
-            let timer = {
-                let mut timers = self.timers.borrow_mut();
+            let mut timers = self.timers.borrow_mut();
 
-                if timers.is_empty() || timers.last().unwrap().next_call > base_time {
-                    break;
-                }
+            if timers.is_empty() || timers.last().unwrap().next_call > base_time {
+                break;
+            }
+
+            timers_to_run.push(timers.pop().unwrap());
+        }
+
+        for timer in timers_to_run {
 
-                timers.pop().unwrap()
-            };
             let callback = timer.callback.clone();
 
             // prep for step 6 in nested set_timeout_or_interval calls